Diode charging supercap solar battery

Summary

This post investigates the ideal diode my Maxim Integrated, part MAX40203, as a possible replacement for low forward voltage diodes.

Example Diode SuperCapacitor Charger

Forward Voltage
When deciding on a suitable diode for a circuit, such as the basic diode supercapacitor charger shown above, a Schottky is a usual choice. The lower forward voltage of the Schottky diode is more beneficial to ensure that the load operating voltage is closer to the supply voltage. There is also the benefit of lower losses as a result of the lower forward voltage.

Some examples of different Schottky diodes include the Toshiba CUS10S30 with a voltage drop of 230mV at 100mA, the Panasonic DB2S30800L has a drop of 420mV at 100mA or the Nexperia PMEG10020 with a drop of 500mV at 100mA.

Ideal Diode
Released in the middle of 2018 the MAX40203 is targeted as a replacement for the Schottky diode and it does not disappoint in regards to forward voltage.

MAX40203 - Courtesy Maxim Integrated

Diode Testing (Reverse leakage)
To begin the tests, reverse leakage was measured. The MAX40203 was bench tested against two general Schottky diodes, the Nexperia PMEG10020 and an ST STPS2L40U. To perform tests with the Schottky diodes, the devices were connected in reverse bias with a 100k resistor. The MAX40203 leakage test was performed with 100k resistors to measure leakage through Anode and GND as shown in the device datasheet.

MAX40203 Leakage - Courtesy Maxim Integrated

Voltage measurements were made across the resistor as the supply voltage was increased in one volt increments. Since the maximum operating voltage of the Maxim part is 5.5VDC the test voltage was limited to 5V.

Reverse leakage Schottky vs MAX40203

Graphing the above table of results was certainly not necessary although illustrates the leakage difference between devices. Note the reverse leakage on the PMEG diode is magnitudes lower than the Maxim part. At 5V DC the PMEG diode leakage was 30nA compared to the 207nA for the Maxim part.

Graphed reverse leakage Schottky vs MAX40203

Maxim Part Enable
The MAX40203 datasheet does state that the Enable pin should be pulled high however it also states that there is an internal weak pullup.

Reverse leakage tests were performed with only the Maxim device and the leakage through the Anode was measured. Once again the power supply voltage was increase in a range of 1C to 5V DC.

MAX40203 Reverse Leakage Test Setup

These tests were to replicate a circuit, such as the example above, using a solar panel.

MAX40203 Enable On/Off Reverse Leakage Measurements

Some difference in measurements was noted when the Enable input was connected to the supply. 

Diode Testing (Forward Voltage)
The MAX40203 was subsequently tested with the diodes from the previous test in forward bias. Resistive loads were changed with a fixed supply voltage of 5V DC to achieve test currents from 1mA to 1A.

Forward Voltage Schottky vs MAX40203

Graphing the above data illustrates the usual curves for diode forward voltage with the almost linear voltage drop against forward current across the MAX40203 internal FET.

Graphed forward voltage Schottky vs MAX40203

MAX40203 Load Testing
Measurements were taken to verify the forward voltage of the Maxim part against the device datasheet to a current of 1A. These were similar to the specifications for a room temperature of 25°C and not recorded.

For the final set of tests the MAX40203 was powered up and down with varying resistive loads with a fixed supply voltage of 5V DC. A repurposed board served as the carrier for the test device.

MAX40203 Test Setup

Tests were performed with various wirewound resistors and initially the power supply current limited to 1A. Final tests were conducted with a current limit at 2A.

MAX40203 Load Test Results

The first three tests shown in the results above were relatively normal. For the last test with a 0.1Ω resistor the power supply current limit was increased to 2A. After power was supplied to the device, it warmed considerably then the current reduced to around 180mA so power to the device was removed. The internal protection was suspected to be active. After cooling the device did not output the supply voltage of 5V, instead it was around 4V DC. Furthermore the quiescent current was 32mA which had also increased.

MAX40203 Short Testing
A new MAX40203 was placed on a new test board as the device from the prior test was suspect. In the last test the output of the device was shorted to 0V to test the short circuit protection.

After applying power the supply showed that the device was passing and holding 1A. The current limit on the supply was then increased to 2A, still ok, then 3A; after 3A the current dropped to several hundred milliamps. The device was allowed to cool but never returned to normal operation.

Comments
Testing showed that the low forward voltage drop of the MAX40203 makes it ideal for specific charging applications. For a charging current of 100mA the test Schottky's 481mV was over ten times larger than the Maxim devices 35mV.

Conversely the reverse leakage of the Schottky 30nA was significantly lower than the 316nA for the Maxim device.

Testing of the Maxim device short circuit protection was incomplete and would warrant additional further review.

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!