How to Hack your DC/DC Converter – Part 1

This article will use the example of an isolated gate driver power supply to show four possible DC/DC converter ‘hacks’ that could be used for many other applications. It is based around the RxxPxx series of converters from RECOM which have 6.4kVDC basic isolation (250VAC working voltage), industrial operating temperature range and a low isolation capacitance; thus, they are particularly suitable for powering high voltage isolated gate driver circuits.

Important note: Using any electronic component outside of its intended operational range will invalidate its warranty. If you decide to use any of the following solutions, please contact the manufacturer and get their agreement in advance.

Suppose you have a SiC transistor application that requires a positive gate drive voltage of around +15V and a negative gate drive voltage of around -4V for optimum performance with the lowest switching losses (Figure 1). You check the manufacturer’s datasheets and find that an isolated DC/DC converter with this particular asymmetric output voltage combination does not exist as a standard product. What can you do?

 

Figure 1: Simplified schematic of an isolated SiC transistor gate driver circuit. Image used courtesy of Bodo's Power Systems.

 

Hack #1: Use an Out-of-Specification Input Voltage

Low power unregulated DC/DC converters typically have an output/ input voltage variation ratio of 1.2%/1% of Vin, when measured at full load. In other words, if the input voltage is 10% above or below the nominal value, the output voltage will be around 12% too high or too low, so any variations in input voltage are amplified. The voltage ratio improves linearly with reducing load; hence, at 50% load, it is around 1.1%/1% of Vin and at minimum load it is roughly parity (1%/1%). To allow comparison between different manufacturer’s datasheets, the electronics industry has standardised on line regulation specifications for unregulated converters to be over a standardised ±10% variation in the supply voltage. However, what happens if the input voltage is set outside of this range?

The answer is that the converter will still function, but the performance parameters are no longer guaranteed by the datasheet specifications. If the input voltage is too low or too high, the output voltage will be also be too low or too high and, therefore, out-ofspecification, but his might be very useful in certain circumstances. The following images (Figure 2) and test results (Table 1) show measured circuit voltages using the RECOM R-REF01-HB evaluation board driving a SiC MOSFET with a 1MHz PWM switching signal:

 

Figure 2: Test results with a R12P22005D DC/DC converter with isolated +20/-5V outputs with a nominal 12V input (left) and 10.8V supply (right), both measured under a realistic 60% load, powering an active SiC transistor gate driver. Image used courtesy of Bodo's Power Systems.

 

Table 1: 2W isolated asymmetric output voltage DC/DC converter (R12P22005D) with nominal 12V, nom-10% and out-of-spec nom-20% supply voltages, measured with a 60% load. Image used courtesy of Bodo's Power Systems.

 

As can be seen from this hack, setting the input voltage to -20% below nominal (9.6V) results in the desired non-standard output voltages, even though the input voltage is outside of the datasheet specifications. How far can we go with this hack? Well, let’s see:

 

Table 2: 2W isolated asymmetric output voltage DC/DC converter (R12P22005D) with nominal and out-of-spec under-voltage supply voltages, operating with 60% load (line reg. ≈ 1.1%/1% of Vin). Image used courtesy of Bodo's Power Systems.

 

In the other direction:

 

Table 3: A 2W isolated asymmetric output voltage DC/DC converter (R12P22005D) with out-of-spec over-voltage supply voltages, operating with 60% load (line reg. ≈ 1.1%/1% of Vin). Image used courtesy of Bodo's Power Systems.

 

We can see that the converter does not suddenly stop functioning even when the input voltage is way outside of the ±10% limits provided in the datasheet.

Caveat: Operating the converter outside of its specified input voltage range will increase the internal stresses on it, so other specifications provided in the datasheet such as efficiency, output ripple and operating temperature range may not be met. If the input voltage is very low, the increased input current could cause the primary side components to overheat. If the input voltage is too high, the voltage ratings of the internal capacitors and transistors may be exceeded. Both out-of-spec conditions are likely to allow the output voltage to drift significantly with changes in ambient temperature or load; therefore, use this hack with caution! For a more dependable solution to the problem of generating a non-standard +15V/- 4V asymmetric output voltage combination, we need to use a semiregulated circuit:

 

Hack #2: Regulate Just One of the Outputs

Using an asymmetric output DC/DC converter such as the RxxP21509D with +15/-9V nominal outputs, if one output voltage is correct, the other can be easily post-regulated down to the desired output voltage. For our isolated gate driver power supply example, the negative rail output current is lower than the positive rail current; thus, a Zener diode with a general purpose NPN bipolar transistor regulator solution can be used (Figure 3).

 

Figure 3: Negative rail regulated solution. Image used courtesy of Bodo's Power Systems.

 

The advantage of this solution is that the DC/DC converter is operating within its datasheet specification range, so both the performance and warranty are unaffected, including operation over the full industrial ambient temperature range of -40°C to +85°C without derating.

Additionally, the negative rail is now regulated and remains fixed and independent of variations in load or input voltage and can be set to any desired voltage within range by choosing a different Zener diode voltage. The same technique can be used to regulate the positive rail if this is more critical than the negative rail (see Hack#3).

The disadvantage of this hack is that the regulated rail current is limited by the power dissipation in the transistor. In this example, the NPN transistor needs to drop around 5V across it, which would limit it to a maximum of -100mA average load current (note: the peak gate charge/discharge current is supplied from the output capacitors, so only the average current drain needs to be considered here).

If more output current is needed without incurring excessive heat dissipation, stacked converters would be a better solution:

 

Hack #3: Stacked Converters

The average power taken by a gate driver is dependent on the gate drive voltage swing, the gate charge of the transistor and the switching frequency; it can be approximated by the following equation:

 

\[P_{gate~driver} = V_{gate}Q_{gate}ƒ_{sw}\]

 

Therefore, more gate drive power is needed when switching at higher frequencies or when driving paralleled gates to increase the output current, for example. However, as the gate drive voltage in our example is asymmetric (+15V/-4V), more power is needed on the V_gate positive swing than the negative swing. If the power consumption exceeds the capability of a single isolated DC/DC converter, two different stacked converters can be used (Figure 4). The following hack delivers +16V @ 2W and -5V @ 0.7W:

 

Figure 4: Stacked DC/DC converters. Image used courtesy of Bodo's Power Systems.

 

The R12P209D dual-output DC/DC converter is used with the common pin disconnected, creating an unregulated 18V/222mA supply. This is then regulated down to the +16V VDD rail by the Zener and NPN transistor combination. As the NPN has double the current, but only has to drop half the voltage across it, the transistor power dissipation is about the same as in Hack#2.

Additionally, the 5V linear regulator for the gate driver non-isolated primary side has been replaced with a low-cost R-78E switching regulator module capable of delivering 5V at up to 500mA. This supplies both the gate driver primary side and the R05P05S DC/DC converter used to provide the isolated -5V output rail, meaning that any variations in the 12V supply voltage are now regulated out in the negative rail. Operating an unregulated DC/DC converter from a regulated supply improves the overall system performance and is the basis for the next hacking idea: using cascaded converters.

 

Hack #4: Cascaded Converters

As we have seen from Hack#1, the output voltage of an unregulated DC/DC converter can be ‘tweaked’ by adjusting the input voltage. If an adjustable output isolated asymmetrical gate driver voltage is needed, adding low-cost, non-isolated and pre-regulator DC/DC modules can create a gate driver circuit that can be set to be within a wide range of gate voltage values. This hack is useful to test and check which combination of positive and negative drive voltages offers the highest performance with the lowest losses. Afterwards, fixed voltage trimming resistors can be fitted to set the output voltage combination that works best:

 

 

Figure 5: Adjustable asymmetric output isolated gate driver power supply. Image used courtesy of Bodo's Power Systems.

 

The RPX-1.0 is a particularly useful low-cost SMD DC/DC module, as it offers a very wide output voltage adjustment range (0.8 – 30V), with an impressive 1A continuous output drive current. The output voltage can be pre-set using two resistors or, as shown in this hack, made variable by using a trimmer resistor.

As with all hacks, using any product outside of its intended use means that caution is recommended, even if the motto “If it’s stupid and it works, then it’s not stupid” holds true. If in doubt, contact RECOM technical support. We can test the solution and advise on suitability in your particular application. For volume production, we also offer modified standard converters with any desired input and output voltage combinations, so you can get a semi-custom solution with the full manufacturer’s warranty.

 

This article is the first part of a two-part series, the second being “How to hack your AC/DC converter”

 

This article originally appeared in Bodo’s Power Systems magazine.