CUI INC PDQ15 Remote Control Pin

Summary

This blog illustrates the measurement of the remote control pin available on the CUI INC® PDQ15 series of brick power supplies. Similar tests were conducted in a previous post with two MeanWell supplies.

CUI INC PDQ15 power supply

Control Pin
From the CUI INC PDQ datasheet (page 2), the voltages required to drive the ON/OFF pin are listed as greater than 3.5V for ON and less than 1.2V for OFF.

Remote control pin specification

Control Pin Input Voltage Measurements
To verify the operation of the control pin on the CUI INC PDQ converter, two outputs of a Rigol DP832 linear regulator power supply were used in the same manner as a previous blog which used MeanWell converters.

Rigol DP832

An initial test was conducted to determine the ON / OFF voltage hysteresis for the converter. Turn ON was 8.7V DC and turn OFF 8.0V DC. Tests were conducted with no input on the remote control pin (floating).

For the second test, one channel of the power supply provided the input supply to the DC to DC converter and the second channel on the supply provided the supply for the control pin. Both outputs of the power supply had a common 0V connection.

PDQ bench testing

The supply input voltage was increased, from the threshold ON voltage and the control pin threshold voltages determined and recorded.

Measurements for the CUI INC PDQ15 is listed below.

PDQ Measurements for supply vs control pin voltage

A visual representation of the above data is shown below.

PDQ Graphed measurements

The graph shown above is scaled to show the small variation in voltages required to activate the converter using the remote control pin.

Note On Temperature
The cold body temperature of a PDQ15 converter was measured at 19.2C. With a 24VDC input supply connected, no load, the PDQ15 has a quiescent power of just less than 2W. After 30 min the body of the supply had raised to 57.1C.

Summary

For this test of the characteristic response of the PDQ15 remote control pin, the measurements showed that it was similar to the MeanWell SKM series. The remote control ON/OFF voltages were flat after 14VDC although the voltages were not similar to the MeanWell SKM or SDM converters.

MeanWell SDM SKM Remote Control Pin

Summary

This blog highlights the need for verification and validation testing when changing or updating electronic parts. In particular all facets of the device should be reviewed or tested whenever possible.

Background
In a commercial arena, new parts or those parts earmarked for an upgrade, are usually checked on paper for suitability. The time spent on the comparison between parts usually depends on the parts complexity although specifics can be inadvertently overlooked from time to time. After the new part is sanctioned for use it is commonly bench tested or bolted onto an existing design to validate operation before being included in a new hardware (PCB) design.

For this blog two MeanWell products are referenced, the SDM and SKM series of DC to DC converters.

 MeanWell SDM30-24S5

MeanWell SKM15

Operational Characteristics
Comparing the datasheets of the MeanWell SKM to the SDM series, the SKM is technically the better device. After all, the SKM is newer technology and it performs accordingly during bench tests.

One technical aspect of these converters which can be overlooked is the hardware difference between the inputs used to switch the converter ON and OFF. For the SDM this input is called an ON/OFF pin and for the SKM it is called RC for Remote Control.

SDM converter block diagram

The block diagram shown above illustrates in block diagram format how the control pin for the SDM converter enables or disables the PWM portion of the circuit. A similar block diagram for the SKM was not available at the time of writing this blog.

Control Pin Difference
From the MeanWell SDM datasheet, the voltages required to drive the ON/OFF pin are shown as 5.5V ON and 2.5V maximum for OFF.

SDM ON/OFF control voltages

The MeanWell SKM datasheet lists the voltages required to drive the ON/OFF pin as greater than 2.5V ON and 0.5V maximum for OFF.

SKM ON/OFF control voltages

This information is clear enough, well vague enough! Other factors also need to be considered depending on the method used to drive the control pin. These could include, type of input, input voltage range, current required to drive the input, absolute maximum ratings and response times / delays where applicable.

For this blog only the input voltage range of the control pin will be looked at in detail.

Control Pin Input Voltage Measurements
To verify the operation of the control pin on the MeanWell SKM and SDM converters, two outputs of a Rigol DP832 linear regulator power supply were used. 

Rigol DP832

An initial test was conducted to determine the ON / OFF voltage hysteresis for SDM model, 100mV, and the SKM model which had a hysteresis of 2000mV. Tests were conducted with no input on the remote control pin, floating.

MeanWell SDM, SKM supply voltage hysteresis

For the second test, one channel of the power supply provided the input supply to the DC to DC converter and the second channel on the power supply provided the supply for the control pin. Both outputs of the power supply had a common 0V connection.

This is one method of testing the MeanWell supplies. There are other methods that could be used to drive the control pin although a voltage derived from the actual input supply appeared to be a common method for controlling this input pin.

SDM bench testing

The supply input voltage was increased, from the threshold ON voltage and the control pin threshold voltages determined and recorded.

SKM bench testing

Measurements for the two MeanWell supplies are shown below.

SDM Measurements for supply vs control pin voltage

SKM Measurements for supply vs control pin voltage

A visual representation of the above data is shown below.

SDM Graphed measurements

SKM Graphed measurements

Summary

When migrating between MeanWell SDM and SKM DC DC converters, reviewing the design should be performed as a matter of good design principle. 

For designs using the ON/OFF control pin, the drive circuit should be reviewed for a suitable drive level under a range of operating conditions.

At a supply voltage of 24VDC, the SDM series start operating at 3.4VDC whereas the SKM starts operating significantly lower at 1.2VDC. This of course sounds like a better hardware feature. The SKM supply could be driven from designs using lower circuit voltages, however narrowing the operating gap between 0V common and supply has its own drawbacks. Noise immunity, ESD or inadequate earthing can each introduce issues into the design if not considered carefully at design time.

Salvaging electronic parts - Part 1

Summary

Prompting this blog was my dismay which was caused during a recent scrounge through some component draws for a humble load resistor. The culprit is shown below, just an innocuous resistor right!

Old resistor

Before using the resistor a standard multimeter check was performed. This measurement indicated that the resistance was unexpected high resistance. After pulling lightly on the two legs of the resistor simultaneously the reason for the high resistance was apparent, mechanical failure. This faulty resistor would have been moved around in a draw of spare parts for the better part of ten years before it was used, possibly adding to its demise.

Old faulty resistor

Whether mechanical failure of the resistor occurred during manufacturing or more likely as a result of being mistreated during storage, this highlighted the limited shelf life for salvaged or even new electronic parts. Certainly some electronic parts are more susceptible to the rigors of handling, storage damage and part aging.

Salvaging Parts
This blog and subsequent blogs on the same subject, intend on showing some of the parts than can be salvaged from electronic equipment and possible issues that can be experienced with these parts. Whilst not all factors that damage electronic parts has been accounted for, the factors mentioned should serve as a guideline for those performing salvaging themselves.

A fully functioning control board from a NESS security system (D8X D16X) will serve as the example. For any board being salvaged a check with a multimeter can save the hassle of removing an already faulty component.

Ness alarm control board

The NESS control board has had the heatsink situated within the blue box shown above, however this was removed for visibility of surrounding components. In fact this heatsink would be considered the first salvaged part!

Power Supply

Within the bounds of the blue box shown in the image, are parts associated with a linear power supply. Parts to be salvaged could include the red varistors (V56ZA05P), DIP bridge rectifier, large electrolytic, linear regulator and associated surface mount capacitors.

Microcontroller

The red box identifies the alarm panel processor and another smaller power supply. Due to the age of the alarm panel board the processor, a Cypress Semiconductor MB89F538, this part is listed a obsolete which may preclude it being removed. The remaining crystal, fuse, DIP8 regulator, surface mount capacitors and logic level FET - BUK9245 could all be removed.

Line Out
For the telephone interface, shown by the yellow box, are parts such as optocouplers, a relay and an LMV324 rail to rail op amp. All these parts can be readily used in new projects.

Input Section
The input section has not been particularly noted for parts to salvage although the MELF and larger surface mount resistors come in useful for higher voltage designs. Of interest in this section is the PCB design itself and use or lack of components. More on spark gaps in a following blog.

Sections Not Noted
In the remaining sections are more logic level FET's BUK9245, ULN2803 driver and connectors would also come in useful when designing.

Salvaging Equipment
The tools required for removing salvaged electronics will vary depending on the equipment and budget. For most types of salvage work equipment such as needle nose pliers, SMT tweezers, a second gas or mains powered soldering iron and a range of drivers for different types of screws and bolts. On occasion equipment designers use TORX or HEX bolts to secure equipment.

As an example a computer power supply, which is commonly a single side circuit board, can be stripped of electronics using a soldering iron and a solder sucker or wick.

For circuit boards with tracks on both sides, with plated thru holes, desoldering can use more consumables such as solder wick especially for parts with multiple legs.

Surface mount components can be removed using additional equipment such as a soldering station with tweezers or a reflow oven can reduce issues relating to mechanical stress on electronic parts or over-heating when compared to using traditional solder wick. For the higher end user a hand held air convection tool offers timed temperature control however it is usually out of the price range for most hobbyists.

One noteworthy item not commented enough in other blogs for my liking, is the use of Personal Protective Equipment (PPE) when handling equipment made with lead based solder. This should not be taken as an alarmist comment although viewed as a simple measure to limit ones exposure to lead during the salvaging process.

Ansell cotton gloves

Workpiece Holding
When small to medium sized circuit boards, up to 150mm in width, are worked on for salvaging, a circuit board holder of any variety assists when changing between the component and solder sides of the board. 

Circuit board holder

For the salvaging process with the Ness control board a PCB holder from Altronics, T2356 shown above, was used.

Salvaging Components
To begin with the two and three pin components, with thru-hole (TH) leads, will be removed from the NESS board. As mentioned above these components consist of capacitors, varistors and regulators.

Flooding capacitor pins

Capacitor - Thru Hole
In order to transfer more heat from the soldering iron to the pad on the circuit board, adding a small amount of solder to the pads, of the part being removed, usually helps. The component can then be pulled and rocked out of its original position.

Capacitor removed

Excessive pulling force applied to the leads of the capacitor whilst desoldering may lead to the pins being partly lifted from the capacitor body. When a visible change is lead length can be seen, then the part should be discarded. Overheating a leaded capacitor is possible although unlikely in most instances. Many leaded capacitors have the maximum working temperature listed on the capacitor.

Regulator - Thru Hole
A similar method to the capacitor removal can also be applied to three terminal regulators such as the 78xx series.

Flooding regulator pins

As with the capacitor, it should be noted than one or more of the regulator pins may be connected to large PCB tracks, fills or entire planes on the circuit board. These larger areas of copper require more heating to melt the solder, so flooding the pins with some extra solder can facilitate a better transfer of heat from the soldering iron.

Regulator removed

Post removal the regulator, and similarly any other removed parts, would benefit with clean-up to remove the excess solder from the pins. A 78xx series leaded regulator from ST has a storage temperature of 150C, damaging these robust parts is also difficult.

Relay- Thru Hole
A through hole relay, which usually has 6 or more leads, can be removed with the old favourite desolder braid or a solder sucker.

Desoldering through hole relay

While using desolder braid to remove any electronic component, a fan or fume extractor is strongly recommended. Well known brand Weller has an article "Health Hazards From Inhaling and Exposure To Soldering Fumes" describing the risks of inhaling fumes created as a result of desoldering with braid.

Desoldered relay

One benefit of using the desolder braid is that during the process of desoldering, while the solder is being wicked from the solder joint, the position of the soldering iron tip can be moved around the component pin. This allows for cleanly desoldered pin and prevents the pin from sticking to the wall of the pad.

Surface Mount Components
Removing a surface mount component can pose its own difficulties. Some components can be glued, part of the reflow soldering process, or held to the board with Silastic for mechanical stability. In these instances removal of the component may require some more inventive solutions.

Capacitor - Surface Mount
For removal of a surface mount capacitor the soldering iron tip can usually be applied directly to the exposed pad of the capacitor to melt the solder and the component leg lifted away from the circuit board pad.

 Surface mount capacitor

To remove smaller capacitors, each of the exposed legs may need to be heated more than once and the part rocked free. This method is recommended because beneath the capacitor is a plastic former which holds the component flush with the circuit board. This plastic former can be melted if the soldering iron is kept on the component leg for several seconds.

Removed surface mount capacitor

Opto-Coupler - Surface Mount
For removal of a surface mount opto-coupler the soldering iron tip can used to heat the two circuit board pads located on one side of the component. There is usually more solder on the opto-coupler pads than components such as resistors and capacitors so additional solder should not be required.

Opto-coupler tweezer removal

A tip for removing these types of devices is to insert the arms of some stainless tweezers beneath the component. When the two pads on the same side of the device are heated, only a small amount of force is needed to lift the component from the circuit board.

Opto-coupler removed

The same technique is applied to the remaining fixed side of the component. Using the tweezers also allows removal of the heated component without the need to touch it directly.

MOSFET - Surface Mount
The last part to removed in this blog is a MOSFET in a DPAK case type. As with the opto-coupler using tweezers can be helpful to lift the component legs.

Desoldering leg of MOSFET

With no additional solder and the soldering iron applied to the leg of the MOSFET, only a small amount of force is needed to lift each component leg from the circuit board using the tweezers. It may also pay at this stage of the removal to use desolder wick to ensure that the legs of the MOSFET are in no way connected to the circuit board.

To heat the soldered tab of the MOSFET, adding some solder to the tab joint aids in heating the surface mount pad and MOSFET itself.

Heating MOSFET tab

Heating the device and the pad may take several seconds. When the solder near the tab begins to melt, applying horizontal force to the component usually allows the part to be slid off the pad. Heating the tab of the MOSFET for an extended time should be avoided.

Desoldered MOSFET

The maximum time a component should be heated can be based partly on the soldering process reflow curve. This curve is not always available from manufacturer data sheets especially for discrete devices so the peak temperature could also be used.

For the MOSFET that was removed from the NESS circuit board, BUK9245-55A, the device is several years old and the MOSFET 'storage temperature' of 175 degrees the only details listed.

BUK9245-55A Characteristics

As a general rule of thumb the solder reflow peak temperature should not be exceeded for more than 30 seconds. Applying this to the MOSFET removal, with a 260 degree soldering iron set temperature, then it should be safe to use 15 seconds of heating on such a device.

Finally as with any component it should be tested thoroughly before entering the confines of any spare part storage system!

Paralleling Diodes with Resistors

Summary

This blog demonstrates how diodes can be connected in parallel with a suitable series resistance per diode. These current sharing resistors are required for each paralleled diode primarily to handle the negative temperature coefficient (NTC) effect on diode forward voltage.

Using resistors with multiple diodes may be required for a number of reasons such as design costs, package size, availability of required diode(s) or something as novel as limited space on a printed circuit board (PCB).

The method using current sharing between diodes is particularly useful for designs where space is a premium and adding a thermal mass between diodes is not possible.

Literature
There is documentation relating to paralleling diodes although it can be difficult to locate online. ST Micro published an Application Note AN599: Parallel Operation of Power Rectifiers in 1993 and Application Note AN4381: Current Sharing in Parallel Diodes.

NXP also have a similar Application Note AN11358: Parallel Schottkys as secondary rectifiers in flyback adapter.

Diode Matching
Diodes bearing the same part number are made with similar characteristics however devices are never identical. Slight variations in diode characteristics such as forward voltage 'Vf', during parallel diode operation, can result in one diode sharing more current than others.

Diode matching, similar to LED binning, can be performed through a process of measurements targeting diode forward voltage drop for various currents (VI curves). Attention should also be made to the diode temperature. The solution of matching diodes may be suitable for low volume specific designs or product prototyping however this is not suited for volume (mass) production.

Diode Testing - No resistance
To illustrate how one diode, in a two diode parallel setup, will eventually end up at a higher temperature than the other a bench test was conducted.

Dual Schottky diodes in parallel

Two Schottky diodes (ON Semiconductor parts) were soldered in parallel. A 1.8R 5W resistor was used to limit the power supply current. 

Temperature of one Schottky Diodes when operated in parallel

Temperature measurements were performed at regular intervals using a FLIR C2 thermal camera and the power supply used for testing was a Rigol DP832.

The power supply voltage was under 2.5VDC. The total power supply power was limited to 1.3W.

Temperature Measurements: Schottky Diodes no resistor 

As shown in the table above, one of the diodes began to heat more rapidly in under thirty seconds. After four minutes the test was not continued because the diode junction temperature Tj was approaching the devices specified rating of 125C.

Diode Testing - 0.01R resistance

For the second test the two diode setup was modified to include a 0.01R resistor on the cathode of each of the diodes. This resistor is the standard type used for current sensing.

These resistors were added to test if a small resistance, such as a PCB traces or the resistance in leaded components, would balance the current between the diodes.

Dual Schottky diodes in parallel with serial 0.01R resistors

The power supply power was again limited to 1.3W and temperature measurements conducted.

Temperature Measurements: Schottky Diodes 0.01R resistor

The results above show that the diodes began to heat at a more consistent rate compared to no series resistance until five minutes. After six minutes one of the diodes had begun to heat more than the other and was drawing most of the current.

This test indicated that such as low resistance was insignificant and unable to balance the drop in diode forward voltage caused by the heating of the diode.

Diode Testing - 1.3R resistance

The two diode setup with resistors was modified to use a 1.3R resistor (0805) on the cathode of each of the diodes. These resistors values were guided by the formula shown below.

Measuring each of the diodes forward voltage Vf with a digital multimeter, diode 1 read 0.64V and diode 2 read 0.65V. With approximately 250mA being equally passed by each diode the series resistance calculates to 2.6R, this value was halved to reduce the resistor power dissipation and tested.

Dual Schottky diodes in parallel with serial 1R3 resistors

Resistors with the case size 0805 were fitted as shown in the image above.

Temperature Measurements: Schottky Diodes 1R3 resistor

The test results showed the diode temperature stabilized after two minutes.

The above results however were a brief snapshot of a lengthy soak test which did show that some variation in diode temperature did eventuate. The difference in the temperature of the two diodes was stable and no runaway was seen during testing.

Temperature of one Schottky diodes when operated in parallel with series resistance

When viewed thermally the balance between the diodes can easily been seen when compared to the test where no series resistance was fitted.

Current Sharing - 1.3R resistance

Using a multimeter the forward voltage across the resistors was measured after 10 minutes. One resistor showed a voltage drop of 0.374V and the send 0.363V. 

Using the voltage across the resistors to determine currents showed that 0.289A and 0.279A flowed through each resistor diode legs.

It should be noted that the current through each resistor diode leg is dynamic and should never be considered exact. Additionally the current through each leg should not be thought of as balanced. When the third test was allowed to run for a considerable time a temperature delta of 10 degrees was often seen between the two diodes.

Summary
For the tests conducted in this blog, with a pair of ON Semiconductor Schottky diodes, adding a suitable value series resistor to each of the diodes facilitated current sharing, without the need for thermal bonding.

Fitting the two diodes to a common thermal mass or thermally coupling the diodes may also be another solution to balancing the current sharing although not investigated in the scope of this blog.

Additionally the tests employed a benchtop DC power supply as the source. Other supplies such as AC or pulsing supplies (PWM) with shaped waveforms and parallel diodes may require more specific testing to determine operational characteristics for a parallel diode design.

Design Suggestions
Listed below are some design suggestions when using diodes in parallel.

1. Choose diodes with the lowest forward voltage (Vf) that can be afforded for the design. This aids in reducing heat dissipated which is especially a concern for enclosed spaces.

A single bigger diode is not always a better solution if it is dissipating significantly more heat.


2. For the current and voltage rating of the diode, add an appropriate safety margin suitable for the design.


3. A design with parallel diodes operating with continuous DC current may experience less stress than parallel diodes operating as freewheeling (flyback) diodes for inductive loads and therefore should be rated accordingly.


4. For the current sharing resistor select an appropriately rated device for power dissipation. When designs use PWM at high currents the resistors may also benefit from being rated for pulse withstanding.


5. Test, test and test. Testing does not refer to the product operational or consumer specifications but the working limits of the design. To understand your product it should be tested to failure!