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.
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.
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| MeanWell SDM30-24S5 |
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| 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.
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.
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.
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.
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.
The supply input voltage was increased, from the threshold ON voltage and the control pin threshold voltages determined and recorded.
Measurements for the two MeanWell supplies are shown below.
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.
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| 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.
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.
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| 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.
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| 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.
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| Rigol DP832 |
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| MeanWell SDM, SKM supply voltage hysteresis |
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.
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| SDM bench testing |
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| SKM bench testing |
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| SDM Measurements for supply vs control pin voltage |
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| SKM Measurements for supply vs control pin voltage |
A visual representation of the above data is shown below.
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| SDM Graphed measurements |
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| 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.
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