PWM driver

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7

PWM driver

As already mentioned, we still need to correctly drive the gate of ESBT. A simple PWM

driver can both drive the gate of ESBT and supervise the fly-back operations requested by

the converter. The common UC3842 provides a cost effective current mode control.

As already mentioned in Section 2.2: Fly-back transformer design, the UC3845 needs 15V

for its correct biasing and this voltage reference is supplied by the auxiliary output of the

transformer. Neglecting for the moment the start-up circuit, next step is to correctly design

the network for the PWM driver terminals.

Calculation and considerations made on the following steps are related to the complete

schematic diagram of the realized demo board reported in Figure 11.

7.1

Primary side regulation

It can be achieved in a simple way just fixing a voltage divider. The advantages of primary

side regulation are essentially linked to its simplicity and low cost, by the way it cannot

supply an accurate control on output voltage at load variation. It is generally chosen for low

power SMPS. The voltage reference of the internal error amplifier is set in the UC3842 at

2.5V, consequently the values chosen for the network resistors are:

●

R12=22 k

Ω

●

R13=3.9 k

Ω

7.2

Control loop compensation

Being the solution adopted a current mode control in discontinuous mode, a simple loop

with just one pole can be used for its simplicity. For this loop we have:

●

R10=150 k

Ω

●

C5=100 pF

7.3

Switching frequency and max duty cycle setting

Since the target of our project is to have f=50kHz and

UC3842 we can set:

●

R15=2.2 k

Ω

●

C6=15 nF

7.4

Current sensing and limiting

Since the inverting input to the UC3842 current-sense comparator is internally clamped at

1 V, we must ensure that the voltage drop on the sense resistor (R18) does not exceed this

●

R18=0.82

Ω