Operating a Switched Mode Power Supply without a Load

Question

Background

I'm a second year student at university, and wanted a simple power supply to test a couple pet projects without going all the way to the lab. I picked up a Power-One MPU150-4350 on the cheap at a second-hand electronics store. The output specifications section seems to state that, for the 3.3V output (V1), 3A is the minimum load while 30A is the maximum.

I know that running a switched mode power supply without a load can produce inaccurate output voltages and even damage the system, although I don't understand exactly why this is the case. However, having to always pull at least three amps from a 3.3V rail seems excessive to me.

Questions

1. What is the minimum load I can put on each of the outputs without damaging the power supply?

2. Will running a switched mode power supply without a load for short periods of time damage it? Or just produce unstable output voltages?

3. Why don't switching power supplies like low currents?

If you only know the answer to one of these please don't hesitate to post. Anything that helps gets +1.

Edit This article will be very helpful for beginners (like me), the answer below is an in-depth and very useful explanation of why SMPSs can fail due to over-voltage when not sufficiently loaded.

Answer 1

It's difficult to generalize this sort of behaviour. Some power supplies will work at less than the minimum load but with degraded performance. Other power supplies may shut down, and others still may malfunction badly (oscillate / shut down). Others may behave perfectly.

Quite often, basic power supplies use pulse-width-modulated (PWM) toplogies with inductive storage elements. The switching frequency is fixed and duty cycle is varied to control the output voltage as a function of load and input.

When the current in the inductive storage element never goes to zero, the converter operates in two states - switch on and switch off. This is called continuous conduction mode (CCM). Once CCM is achieved, the duty cycle essentially doesn't vary (unless the input changes) - the converter behaviour doesn't change with load and things are fairly consistent.

At very light load, there isn't a DC current level in the inductive storage element. The converter now has three states of operation - switch on, switch off and inductor current decreasing, switch off and inductor current = 0. This is called discontinuous conduction mode (DCM). In DCM, output load affects the duty cycle as well as input variations.

Most controllers have a minimum PWM on-time that can be achieved - if the plant tries to command a duty cycle lower than this minimum, you may see erratic output, missing pulses, high ripple current, etc. - some converters will simply stop regulating (the output will rise). Some controllers detect this and go into controlled burst mode to keep the output loosely regulated.

Also, the feedback loop compensation will be dictated by the CCM performance of the converter, since there are nasty things (like right-half-plane zero) in CCM that need to be stabilized that essentially aren't there in DCM - the compensation may be sub-optimal and things like transient response will be affected.

Answer 2

It depends on the PSU design.

Under light or no load a switched mode converter that uses a diode for one of the switches* goes into a discontinuous mode. In this mode for a given duty cycle and input voltage the output voltage will increase substantially as load current decreases.

Most switched mode power supplies are regulated. So as the load is reduced the controller will reduce the pulse width and hence the duty cycle in an attempt to maintain the output voltage.

However as the load is further reduced the pulse width reaches the minimum that the controller can achieve. What happens with very small or zero loads depends on the design of the controller.

1. The controller may maintain the minimum pulse width and duty cycle and allow the output voltage to increase until something goes up in smoke.
2. The controller may maintain the minimum pulse width and duty cycleallow the output voltage to increase until an overvoltage protection circuit is triggered and shuts down the supply until reset.
3. The controller may maintain the minimum pulse width and duty cycle until a self-resetting overvoltage protection is triggered causing wild swings in output voltage as the supply repeatedly shuts down and starts back up.
4. The controller may increase the time between pulses. This allows overall voltage regulation to be maintained down to zero load but it means that the frequency of the output ripple depends on load. This can lead to noise problems both electrical and audible.

My experience is that most modern power supplies fall into category 4 but older designs (which are sometimes still sold) often fell into categories 2 or 3.

Another alternative is for the power supply vendor to build in a "dummy load" to avoid ever reaching the point where the power supply can't reduce the duty cycle any more but I expect that would only be done in specialist applications where output quality is more important than efficiency.

* Converters that use two actively controlled switches (known as "synchronous converters") have the option to remain in continuous mode regardless of load (though at light load discontinuous is more efficient), indeed they can even operate bidirectionally.