How to Hack your AC/DC Converter - Part 2

Let’s assume you have a power monitoring application in an electric vehicle (EV) charging application in which the auxiliary power supply needs to have multiple input options: three phase or single phase AC or high voltage DC or low voltage DC. Additionally, the charging application needs to be able to generate bipolar (±10V) output rail voltages to power the calibrated current and voltage sensors and amplifiers (Figure 1).


You check the manufacturer’s datasheets and find that a power supply with all of these particular input and output voltage combinations does not exist as a standard product. What can you do? You need to start hacking your AC/DC power supply!

Low power AC/DC converters do not normally offer adjustable output voltages. The reason is simple: Unlike, say, an 24-5V isolated DC/DC converter, which might have a transformer with a 5:1 turns ratio, an equivalent AC/DC converter with 230VAC RMS input to 5V output will need a transformer with a 65:1 turns ratio. This is because the rectified AC voltage is significantly higher than the output voltage. The control loop in an AC/DC converter is optimised to compensate for a wide range of input voltages (typically 85-264VAC) and a fixed output voltage. If the output voltage was also made adjustable, the worst case input/output voltage combination combined with the high turns ratio could easily facilitate the converter operation to become unstable.

All of these regulation methods offer the same output voltages with balanced loads. However, there are differences among them if unbalanced loads are used. The advantage of regulating the combined output voltage is that the sum of the negative and positive rails remains fixed, but only regulating the positive or just the negative output offers less variation on that particular rail. There is no universal solution.

However, if switching regulators are used to post-regulate the outputs as shown in Figure 4, both the output voltages remain stable over all load combinations, even down to no load/full load conditions.

There is an additional advantage to using switching regulators on the outputs: They also deliver constant power. Lower the output voltage, the higher the output current. If, in the example of Figure 4, the regulated outputs were adjusted to ±3.3V, then, as long as the overall load was less than 5W, the maximum output current per rail that could be drawn would be up to 1.5A. This is much higher than the AC/DC’s nominal 416mA output current. It is useful because the majority of multi-rail applications need significantly more power on the one rail than on the others (main load + auxiliary loads), but this is not a problem when switching regulators are used. For example, in Figure 4, +12V @ 0.1A, +3.3V @ 1A and -3.3V @ -0.15A would be possible with all of the output voltages tightly regulated.

So far, we have only considered hacking the output of an AC/DC module, but what if we wanted to use alternative power sources? This is about Hack #3.

In a few specific field applications, it is necessary to be able to use either an AC or a DC battery supply, whichever is available. As it can be seen in Figure 7, if an external DC supply was connected to the output of a switched-off AC/DC converter, the output diode, Dout, would stop the external current flowing back through the output winding of the transformer. However, the shunt regulator, IC1, would still be in-circuit. If the external voltage exceeded the set point of the shunt regulator, it would conduct and begin feeding the current through the optocoupler LED.

As the AC/DC converter is inactive, there is no mechanism that would control this optocoupler current. Owing to this, the opto-LED could easily burn out. Therefore, placing an external voltage directly across the output of an AC/DC converter is not advisable. Using two OR-ing diodes could be considered to steer whichever supply voltage is higher through to the application without letting the two supplies interacting with each other (Figure 8).

However, this hack, although simple, suffers from two big disadvantages. Firstly, the output voltage is always one diode drop lower than the supply voltage. Secondly, with higher current loads, the power dissipation in the diodes becomes significant (in the example shown in Figure 8, 3.5W). This means that large, expensive power diodes would be required with possibly additional heat-sinking. Furthermore, the power wasted in diode D2 would harm the battery lifetime.

A better hack would be to use an ideal-diode IC, for example, the LM71300 from Texas Instruments that has integrated FETs for a very compact solution (Figure 9).

This solution has the added advantage: the battery is protected from a damaging deep discharge by the under voltage lockout (UVLO) feature and the application is protected from the high-surge current capability of the battery by the dVdt control. Additionally, the load current from both sources can be monitored using the Imon outputs.

Up until now, the hacks have all been concerned with the output side. This is reasonable, considering that the AC mains is a hazardous voltage and should be treated with the greatest respect. However, returning to our EV charger specification mentioned in the introduction, it can, presumably, function with single phase, multi-phase or high voltage DC supplies. The following hack, consequently, concerns the input side.

The half-wave rectified three phase input circuit shown in Figure 10 will have a rectified DC voltage of approximately 1.17 x Vphase, or about 270V for a nominal 230V single phase voltage. This is extremely high for a standard 230V±10% input AC/DC converter, but acceptable for an AC/DC converter designed to operate with up to 277VAC (the phase-to-phase voltage for 115VAC supplies). The circuit shown in Figure 10 uses input fuses to protect the three phase supply if any of the diodes fail, and the metal oxide varistor (MOV) to absorb excessive voltage surges that could damage the converter. The MOV is optional, as the converter itself is internally fused, but it may be required under local wiring regulations. The input diodes must have a suitable reverse voltage rating.

As AC/DC converters are isolated, the low voltage DC output is floating (galvanically isolated) from the mains supply and can be used as either a positive or negative supply. For example, a common communications supply bus voltage is -48VDC and this can be supplied by any AC/DC with 48V or ±24V output by tying the +Vout pin to 0V and using the –Vout pin for the supply rail.

For some applications, it is either advantageous or unavoidable to ground one of the output pins. At first glance, this would seem a simple enough thing to do, much like the comms power supply application that was mentioned. However, the regulations intended for AC/DC power supplies are not just about safety; there are EMC considerations that also have to be taken into account. Any circulating or induced currents that flow through the insulation capacitance can cause interference, which can make the final application fail the EMC testing. Also, grounding the output is almost guaranteed to cause such an unintended current loop. For instance, for a Class B device, even a loop current of a few tens of micro-amps can push the test results outside of the limits.