SMPS housekeeping electronics power question

Sometimes it's bootstrapped off of the output with a resistor or FET or linear regulator to the line to allow it to startup.

Since that's too wasteful with a big switcher, bigger supplies often have a small auxilliary switching supply that starts up as above and supplies the control electronics of the big guy with a few watts of power.

Best regards, Spehro Pefhany

Many options:

1. Circuit starts on its own. Example: self-oscillating (one or two transistor) oscillator. A small bias current, provided by a large resistor, starts the reaction.

1. Passive. Flyback and forward converters often use a UC3842, TOPSwitch or other controller, which draws little current until supply voltage reaches a threshold. Thus, it can be charged up from zero, using a small bias current from the rectified line. Once the controller starts, current consumption rises, so an auxiliary supply (usually derived from the same transformer) is required.

This is arguably the same as 1., but the circuit is much more complex and won't run on startup current alone, whereas in 1., the circuit operates continuously due to the bias current thus provided.

1. Auxiliary. A separate power supply can be provided (which itself uses one of the above methods, or a line-frequency transformer) which powers the controller. This can provide startup current alone (in which case, another auxiliary method takes over, as is the case for 2.), or it's addutionally used to supply other loads as well. This method is typically used in high power supplies, where the controller recieves feedback and error signals from the output side; drive is coupled to the line side with isolation (usually transformers).

The classical ATX computer PSU uses this method, using a single-transistor oscillator (type 1) to provide standby power (which operates the controller as well as standby functions on the motherboard). The controller is typically a TL494 or variant, using a transformer to couple signals to the transistors (often BJTs).

High power systems often use many auxiliary supplies, so that active power is available *at* the power switch inputs, enabling gate drivers to operate very large transistors. In a power supply with three half-bridge IGBT modules, this requires six separate isolated power supplies, in addition to the controller's power supplies.

Tim

I guess the initial bias current is obtained from the AC power switch-on ? The start-up is the main issue here.

This type of circuit is entirely line operated, so the bias comes from the rectified line.

Although many designs exist, a prominent one is the flyback type blocking oscillator:

Here's a more developed version. Both BJTs and MOSFETs can be used as the main switch, though BJT is more common due to the ease of drive and low cost.

Tim