Derating rectifier diode current

The ratings for rectifier diode currents are often expressed in terms of a steady (ie DC) current, yet when used as a power rectifier with capacitor input filters, the current is very different.

Above is capture of the rectifier input current for a lab power supply set to 1A load current, the scale factor for the current probe is 1V/A.

It can be seen that the current flows for only a small part of each half cycle, peaking at 5A. The RMS value of the current is important as heating of the diodes is partly due to the RMS current flowing in the bulk resistance of the diode.

The RMS value of current shown was calculated and was 2.02A. The average of the absolute value of the samples is the DC current out of the rectifier, and it was calculated at 1.14A, so RMS is approximately 178% of the DC current. The heating effect on transformer windings and diode bulk resistance of the current is 1.78^2 or 3.2 times that of the DC current.

A conservative approach to selection of diodes is to choose them based on the RMS current flowing through them.

An example datasheet interpretation

Datasheets specify the current rating of diodes in various ways and must be interpreted intelligently.

1n4004

The datasheet for a 1n4004 states “Average Rectified Forward Current(single phase, resistive load,60 Hz, TA = 75°C)” to be 1A.

In a bridge rectifier, Iav of each diode is half the DC load current, so these diodes could be used at up to load current of 2A under the conditions stated (resistive load). The form factor for a half wave rectified sine wave is 1.11, so Irms into a resistive load etc is 1.11A, and therefore for the bridge rectifier, Irms is 2.22A. The 1n4004 is barely adequate for the example waveform which is 2.02Arms for 1.1A DC output.

PA40

The PA40 is a 15A rated bridge rectifier.

Above is a derating chart from the PA40 datasheet.

In the example waveform, Ipk/Iav=5/1.14=4.4, we will use the Ipk/Iav=5 curve. Following that curve, it can be seen that at 80° case temperature, the bridge is rated at 11A, derated due to waveform and case temperature.

Note that for discussion purposes, we have used the waveform for 1A load current. In designing a rectifier, you would use the waveform at maximum current, observed using the intended transformer and filter capacitors which all have a bearing on the waveshape.

VKPowermate II 6A PSU

The VKPowermate II design was published in Electronics Australia, a hobby magazine.

The design called for a bridge of 3A diodes and provision was made on the PCB for them. They were renowned for diode failure, followed by serious damage to regulator parts and PCB.

Given this history, I built mine with a PB40 (nominal 25A) bridge.

Above is the rectifier output current, 1A/V. The calculated RMS current for each diode is 4.8A, way too much for a nominal 3A diode (eg 1N5404) and probably near doubling the diode dissipation explaining observed high failure rate of the design.

As an aside, the diodes in the bridge appear to be mismatched with one half cycle contributing more current.

Observation

People are often surprised by the very high temperature of rectifier diodes in operation. Such high temperatures are often the result of failing to properly derate diodes for effects like peaky current waveform and temperature.

An example, the PB40 often appeared in 25A DC power supply projects, and with a capacitor input filter and case temperature of 100°, the current rating is more like 15A (depending on the other components that determine peak current).

Conclusions

Rectifier diode nominal current ratings may need derating for the higher heating effects of waveforms associated with capacitor input filters.