Overcurrent Protection

A number of mechanisms protect the PSU in the event of a short circuit or very high load being placed on one of the outputs.

The PSU has a power limit of around 45W. If an attempt is made to draw more power than this then the PSU will power limit. This will show up as a reduction in the output voltages. This will in turn force the feedback loop to attempt to demand more power throughput to correct the low output voltages. The feedback signal will then trigger the feedback loop failure mechanism which will then force the PSU into latched shutdown if this persists for more than around 1second.

Mains Present Signal

The PSU provides a signal MAINSOK* (active low when mains supply is OK). This signal is provided as an open collector signal from SK8-2. The signal will go high (via a pull-up resistor external to the PSU) when the mains supply is removed and will then provide > around 20mS warning of the imminent collapse of the output rails.

The actual time between the rising edge of MAINSOK* and the output voltage rails falling out of spec is dependent on mains input voltage and output load current. The output voltages will hold up longer when the load current is low and the mains input is high. The 20mS hold up spec is based on low mains voltage and full load.

Inrush current protection

The PSU has an NTC in series with the input reservoir capacitor. When the supply is first powered up the NTC is cold and exhibits a resistance of around 25Ohms which limits the current into the capacitor and prevents damage to the capacitor, fuses and mains switch. After several seconds the NTC temperature rises to around 90C and the resistance falls to a low value allowing normal operation.

PSU Over-temperature Protection

The PSU transformer is fitted with a one shot thermal fuse. This opens at 120C and removes the supply to the primary side controller. The thermal trip is taped to the outside of the transformer so that it could in principal be changed. The materials from which the transformer is constructed are rated for operation at 130C and in normal operation the TX surface temperature is < 90C.

Figure 1

This diagram shows a very simplified PSU block diagram. Only one output is shown and the active clamp is omitted.

The basis operation is as follows.

At the beginning of the switching cycle the gate drive signal to M2 goes high. M2 is turned ON and the voltage across C35 is applied across the primary of L1. During this stage the voltage on the anode of Dout is negative the diode is reverse biased and no current flows in the secondary winding. The voltage applied across the primary causes the current in L1 primary to ramp up from zero. When the control circuit turns off M2 by taking gate drive low then the bottom of the inductor flies back above the input voltage . This flyback action causes the output diode Dout to conduct and current is transferred to the output capacitor Cout.

Figure 2

These waveforms are for a non resonant ordinary discontinuous flyback supply to illustrate the operation of the circuit. The drain waveform starts at the voltage on the input capacitor C35 and falls to near zero when the mosfet is switched on. When the mosfet is switched off the drain voltage flies above the input voltage and produces the ringing waveform as shown. The current then ramps down into Cout. When the current has fallen to zero there is again some ringing. The frequency of the ringing is dependent on circuit parasitics.

The primary current can be viewed by looking at the voltage across R30. The output diode current can be measured by using a current probe around Dout or inferred by looking at the voltage waveform across Cout. The falling ramp should produce a small voltage ramp across the ESR of Cout.

By considering the simplified circuit it can be seen that all of the output rails are developed in the same way. The output diodes provide current to the large reservoir capacitors on each rail.

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Arcam AV8 Overcurrent Protection, Mains Present Signal, Inrush current protection, PSU Over-temperature Protection

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