not support such a load. It is this current, equivalent to

the product of the total gate switching charge (from the

N-channel MOSFET data sheet), times the operating

frequency, that determines the bulk of the MAX5003

power dissipation.

The driver can source up to 560mA and sink up to 1A

transient current with a typical on source resistance of

4

Applications Information

Compensation and Loop

Design Considerations

The circuit shown in Figure 2 is essentially an energy

pump. It stores energy in the magnetic core and the air

gap of the transformer while the power switch is on,

and delivers it to the load during the off phase. It can

operate in two modes: continuous and discontinuous.

In discontinuous mode, all the energy is given to the

load before the next cycle begins; in continuous mode,

some energy is continuously stored in the core.

The system has four operating parameters: input volt-

age, output voltage, load current, and duty cycle. The

PWM controller senses the output voltage and the input

voltage, and keeps the output voltage regulated by

controlling the duty cycle.

The output filter in this circuit consists of the load resis-

tance and the capacitance on the output.

To study the stability of the feedback system and

design the compensation necessary for system stability

under all operating conditions, first determine the trans-

fer function. In discontinuous mode, since there is no

energy stored in the inductor at the end of the cycle,

the inductor and capacitor do not show the characteris-

tic double pole, and there is only a dominant pole

defined by the filter capacitor and the load resistance.

There is a zero at a higher frequency, defined by the

ESR of the output filter capacitor. Such a response is

easy to stabilize for a wide range of operating condi-

tions while retaining a reasonably fast loop response.

In continuous mode, the situation is different. The

inductor-capacitor combination creates a double pole,

since energy is stored in the inductor at all times. In

addition to the double pole, a right-half-plane zero

appears in the frequency response curves. This

response is not easy to compensate. It can result in

conditional stability, a complicated compensation net-

work, or very slow transient response.

To avoid the analytical and design problems of the con-

tinuous-conduction mode flyback topology and maintain

good loop response, choose a design incorporating a

discontinuous-conduction mode power stage

To keep the converter in discontinuous mode at all times,

the value of the power transformer’s primary inductance

must be calculated at minimum line voltage and maxi-

mum load, and the maximum duty cycle must be limited.

The MAX5003 has a programmable duty-cycle limit func-

tion intended for this purpose.

Design Methodology

Following is a general procedure for developing a sys-

tem:

1) Determine the requirements.

2) In free-running mode, choose the FREQ pin pro-

gramming resistor. In synchronized mode, determine

3) Determine the transformer turns ratio, and check the

maximum duty cycle.

4) Determine the transformer primary inductance.

5) Complete the transformer specifications by listing

the primary maximum current, the secondary maxi-

mum current, and the minimum duty cycle at full

power.

6) Choose the MAXTON pin programming resistor.

7) Choose a filter capacitor.

8) Determine the compensation network.

Design Example

< 50mV, settling time

≈ 0.5ms.

2) Generally, the higher the frequency, the smaller the

transformer. A higher frequency also gives higher

system bandwidth and faster settling time. The

trade-off is lower efficiency. In this example, 300kHz

switching frequency is the choice to favor for a small

transformer. If the converter will be free running (not

externally synchronized), use the following formula to

where:

If the converter is synchronized to an external clock,

the input frequency will be 1.2MHz. The external

clock runs at four times the desired switching fre-

quency.

R

kHz

kk

FREQ

SW

=

ƒ









=

100

200

66 7

.

Ω

High-Voltage PWM

Power-Supply Controller

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