AND8098 Datasheet, PDF(3/10 Page) ON Semiconductor – Low-Cost 100 mA High-Voltage Buck and Buck-Boost Using NCP1052

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AND8098 Datasheet, PDF (3/10 Pages) ON Semiconductor – Low-Cost 100 mA High-Voltage Buck and Buck-Boost Using NCP1052

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AND8098/D

output voltage will rise up dramatically and burn the output

capacitor eventually. Hence, a zener diode Z1 or minimum

âdummyâ load resistor is needed to consume the minimum

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

when R1 pulls up the output voltage at a given output current

condition, the output voltages at lower output current

conditions are also pulled up. Hence, the clamping zener

diode Z1 is needed to be with the breakdown voltage as same

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

input voltage Vin and output voltage Vout are related by the

duty ratio D.

Vout

Vin

0.7

The relationship in buck-boost is

(eq. 2)

Vout

Vin

0.7

* 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

Output voltage

Output current

< 0.7 Vin

< 300 mA

Negative & < 2.33 Vin

<< 300 mA, output current is

only a portion of the inductor

current

Input voltage

Operating mode in nominal

condition

< 700 V

Continuous

t 700 * Vout V

t 700 V

Continuous

Standby ability on VCC charging Bad. The current flows through Good. The current passes

current

output even if there is no load through inductor only

Transformer / Auxiliary winding

It is only for standby

improvement or additional

output

It is only for standby

improvement or additional

output

Isolation

Flyback

Depending on transformer ratio

< 10 W. It depends on operating

condition and audible noise level

<< 700 V. It depends on

transformer ratio

Discontinuous

Good. The current passes

through primary winding only

It is a must for the main output.

Additional auxiliary winding can

improve standby performance

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.

Burst mode

PWM

Figure 4. DCM Inductor Currents in Burst Mode

and PWM Control

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