AN3168

Examples of negative power supplies

Doc ID 17193 Rev 1

17/20

For a buck-boost converter the efficiency (as well as the maximum output current) should be

lower and the output capacitor bigger than for a buck converter, as the whole inductor

current is used to charge the output capacitor for the buck converter. But for 230 V AC / 12 V

DC, the duty cycle is very low so there is no great difference between buck and buck-boost

performances. Similar efficiency is reached for both topologies, with the same reactive

components.

3.5

Flyback power supply

The second SMPS topology widely used today by designers is the flyback topology. This

converter uses a transformer to store the energy instead of an inductance. The benefit of

this solution, compared to a buck-boost converter, is the possibility to insulate the output

voltage and also to generate several output voltages by using several secondary windings. A

flyback converter can also deliver a higher power with the same monolithic device (VIPer)

compared to a buck or buck-boost converter.

in implementing an insulated power supply. So only the advantage of implementing a 2nd

low-voltage supply will push designers to use such a topology. Note that this 2nd supply can

then be insulated from the mains.

It is easy to implement a negative supply with a flyback converter, and the output voltage is

insulation has to be implemented elsewhere to protect the appliance user from electrical

shocks (for example, with an insulated keyboard and display).

Note also that due to the transformer ratio, a flyback converter can work with a higher duty

cycle than a buck-boost converter. The input peak current is then lower with a flyback

converter than with a buck-boost converter. The MOSFET peak current is thus lower and so

are its switching losses. The flyback converter efficiency can then be slightly better.

As for buck-boost converter, the VIPer operates in discontinuous mode.

The drawback of the flyback solution is that the transformer used most of the time is a

specific device, whereas a standard inductance can be used with a buck-boost converter.

Consequently, the flyback converter cost could be higher by about 15% than the buck-boost

converter cost for the same output power.

Figure 18 shows the schematic diagram of a flyback power supply using a VIPer16 device.

For a 230 V mains voltage and a 5 V output voltage, the standby power consumption is

typically around 70 mW with this circuit.