Power Supply Design

Question: In what order should you sequence power supply bias power on and off?

Original Question: I am trying to build a SMPS. Having had to repair such PSUs twice, I thought I might try to build one for experience. I am using half-bridge at 25kHz, TL 494 controller, input 240 V AC, output 24 V 30 A DC. I am driving MOSFETS with bipolar transistors through trigger transformers. I have voltage and amps control. To make it easier to trouble-shoot, I am using a separate 24 V DC PSU from the AC supply, for the controller. (03-05-2001)

My questions are:

• In what order should I switch on everything?
• Should the bipolar transistors be on before I start the controller?
• Is the use of soft-start critical?
• Can I just switch everything on together (I suspect not)?

Answer:

Q: In what order should I switch on everything?

A: No system's survival should depend on the order of turning power supplies on or off. Otherwise, if one supply should fail for some reason, have a momentary glitch, or the sequencing logic glitch, then the sequence is changed and the system may fail, resulting in poor field reliability. Making the system immune to sequencing order is one of a designer's design problems. I see this violated so much in recent designs that I repeat it: No system's survival should depend on the order of turning power supplies on or off.

Q: Should the bipolar transistors be on before I start the controller?

A: Yes and no, it depends. When the control is derived from the output, then if the power stage does not start up in the on condition, no power can get to the control circuit and the power supply never starts. If the power stage is on, then the control had better be coming on soon enough so that it has control before the output is reached or the load is damaged by over voltage.

Q: Is the use of soft-start critical?

A: Yes. See my tutorial on the buck converter regarding turn-on and turn-off trajectories in the state-plane.

Q: Can I just switch everything on together (I suspect not)?

A: Since power supplies do not usually have external regulated power to operate from, this is normally how it is done. Some power supplies have a so-called house-keeping power supply to provide regulated power before the main power comes on, but this house-keeping supply has no regulated supplies to operate from. Most non-power supply designers never have to worry about this, so it comes as a shock that they have to design so they get everything from the source or by boot-strapping from the source. Almost every one new to the field designs a power supply that will not turn on for their first design, or design one that turns on and overshoots damaging the load. Always for the housekeeping supply, or for systems that do not have a housekeeping supply, all power is turned on at once and the designer makes sure the control wins the race as power comes up. When the input power is non-monotonic in coming up (relay chatter), then things can get quite tricky.

You have hit upon one of the many design challenges the power supply designer faces.

Others may have a different perspective on these topics. Comments are always welcome.