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What to do when you experience a power failure

Electronic Components and Power Failures:  What happens when electronic components don’t work? Before you run to your local electronics store to buy a new device if the current one is not working, remember that you may just need a few extra electronic parts in order to fix your current device. Below we discuss what happens when your electronics you’ve been tinkering with don’t in fact work.

When working with electronic components, power supply failures can be frustrating, expensive, and time-consuming events. These are highly complex circuits, with many components operating near the edge of their envelope, by design. When they fail, they tend to destroy most of the failure evidence with them, and many times you can spend months on an apparently "simple" problem to track it down to the real root cause. Some common failure mechanisms are detailed below. These are well-known to engineers with many years experience, but most of them are not documented in any easily-accessible form.

Power Supply Failure Mechanisms

Don't Forget Your Safety Glasses!

Think Safety. Power supply circuits are potentially lethal - 400 to 600 VDC is the most dangerous voltage range to humans. At lower voltages, not enough current flows in the body to cause damage. At higher voltages, the current tends to flash over the surface of the body. Never work alone on power circuits.

Even low power, low voltage circuits can be dangerous. Apply a stiff 20 V source to the wrong pins of an integrated circuit, and the results can be dramatic. Always protect your eyes.

Power Switch Overcurrent - either the semiconductor will fail, or bond wires leading to the die will fuse. Always implement pulse-by-pulse
overcurrent protection, whether it's a 10 W bias supply, or a 10 kW inverter. Poor current limiting design is a very common cause of power supply failure. Build the pulse-by-pulse current protection into your very first breadboard, and save yourself a lot of development time in fixing failed circuits.

FET gate overvoltage will cause rapid failure. FETs are very rugged devices (from the right sources - Motorola and IR make excellent parts), they will tolerate very large drain current, and even overvoltage on the drain for short periods. But they will not survive excessive voltage on the gate. Make sure your gate drive circuit can never cause excessive gate voltage, and protect with back-to-back zener diodes if necessary.

High-side silicon drivers are not recommended for high power, high voltage applications. Use a gate drive transformer - they are rugged and provide a negative gate drive for improved noise immunity when the device is off.

Start-Up failures are among the most common problems.
At start up, the output caps are discharged, and look like a short circuit to the power stage. Make sure that your current-limiting is fast enough to survive start up at maximum input line. Do not depend on the soft-start of the PWM controller alone to protect the switches.

FET Anti-Parallel Diode conduction can cause problems.
There is nothing wrong with using this diode in a circuit as long as you do not apply reverse voltage quickly while the diode is conducting. These diodes are very slow, and the subsequent dv/dt rating after conduction is low. It's OK to use this diode for conduction if that particular FET is the next one to turn on - this sweeps out the charge in the diode junction.

A common situation where the diode is inadvertently allowed to conduct is in a full-bridge converter. When a conduction cycle ends, leakage inductance will cause a ringing. If this ringing is large enough, on its first peak, it may reach the dc rail and the FET diodes will conduct. This is OK, these FETs are the ones scheduled to turn on next. However, if the ringing is so undamped that the second peak (in the opposite direction) hits the rail, the other pair of diodes turns on and your circuit can fail at the initiation of the next switch cycle.

To avoid this situation, make sure the snubber provides sufficient damping.

The phase-shifted full-bridge topology inherently controls the FET diode conduction problem better than the hard-switched full-bridge, but it too is not immune from this in some cases.

The current-doubler full-bridge may run into this problem at light loads - be careful to fully characterize the circuit during development to make sure this won't happen.

Make sure your Undervoltage Lockout is working properly at start-up.
Commercial PWM ICs are usually carefully designed to prevent unpredictable operation when insufficient Vcc is applied. However, if you use auxiliary comparators, gate drivers, or other circuits, you have to check them all to make sure they power up properly in a predictable and safe manner.

Large MLC capacitors are prone to failure at high voltage ratings (400 V) and elevated temperature. If you need to use these caps, make sure you test and qualify them thoroughly for all your sources and for your particular manufacturing conditions. Also make sure they are properly stress-relieved for the conditions your power system will see.

Schottky diodes usually fail because of excessive reverse voltage
(assuming they are properly cooled). Make sure you never exceed the voltage rating under any conditions; an 80% derating worst-case is recommended.

Watch out for the full-bridge converter with a dc-blocking cap. If this capacitor becomes charged for any reason, excessive voltage can be applied on one half cycles, and the output schottky can fail very quickly.

The dc blocking cap can also cause an unwanted resonance in the phase-shifted bridge, and can cause the wrong FET diode to conduct. If the control circuit is designed properly, this capacitor, which is large and expensive, should not be needed.

Proper Instrumentation is essential. You need a high-speed storage oscilloscope to capture single-shot events. Power switches can fail in a matter of tens of nanoseconds, and you need to be able to see these failures.

Proper grounding and probe connection of the oscilloscope is also crucial. For high-speed events, remove the ground lead from the scope probes, and connect to the circuit with a very small loop. Differential probes are not recommended.

Hopefully the basic guides from the previous blog posts were helpful in attempting to understand how to get to the root of the problem when you experience a power failure. Be sure to check back often to find more helpful advice from ActiveTech.