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3

output voltage will rise up dramatically and burn the output

“dummy” load resistor is needed to consume the minimum

amount of energy as shown in Figure 1. It is also noted that

condition, the output voltages at lower output current

conditions are also pulled up. Hence, the clamping zener

as the output voltage but it will reduce some of the efficiency

at lower output current conditions.

DESIGN CONSIDERATION

Topology

Buck circuit is to step down a voltage. Buck-boost circuit

is to step up or down a voltage. The output voltage is

inverted. The maximum duty of NCP1052 is typically 77%.

Because of burst-mode control, the effective maximum

duty is lower and said to be 70% roughly. When a buck

converter is in continuous conduction mode (CCM), the

duty ratio D.

Vout

Vin

+ D t 0.7

(eq. 2)

The relationship in buck-boost is

Vout

Vin

+ D

1

* D t

0.7

1

* 0.7 +

2.33

(eq. 3)

Another aspect on topology is the output current. The

maximum output current is always smaller than the

maximum switch current in non-isolated topologies.

However, in isolated topologies such as flyback the

maximum output current can be increased by a transformer.

Table 1. Summary of Topology Difference Using NCP1052

Buck

Buck-boost

Flyback

Output voltage

Negative & < 2.33 Vin

Depending on transformer ratio

Output current

< 300 mA

<< 300 mA, output current is

only a portion of the inductor

current

< 10 W. It depends on operating

condition and audible noise level

Input voltage

< 700 V

t 700 * Vout V

t 700 V

<< 700 V. It depends on

transformer ratio

Operating mode in nominal

condition

Continuous

Continuous

Discontinuous

current

Bad. The current flows through

output even if there is no load

Good. The current passes

through inductor only

Good. The current passes

through primary winding only

Transformer / Auxiliary winding

It is only for standby

improvement or additional

output

It is only for standby

improvement or additional

output

It is a must for the main output.

Additional auxiliary winding can

improve standby performance

Isolation

No

No

Yes. Opto coupler can be

eliminated if isolation is not

needed

Burst-mode Operation

The NCP1052 is with a burst-mode control method. It

means the MOSFET can be completely off for one or more

switching cycles. The output voltage is regulated by the

overall duration of dead time or non-dead time over a

number of switching cycles. This feature offers advantages

on saving energy in standby condition since it can reduce the

effective duty cycle dramatically. In flyback topology, the

circuit is mainly designed for discontinuous conduction

mode (DCM) in which the inductor current reaches zero in

every switching cycle. The DCM burst-mode waveform can

be represented in Figure 4. It is similar to the pulse-width

modulation (PWM) one.

Figure 4. DCM Inductor Currents in Burst Mode

and PWM Control

Burst mode

PWM