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AN701

Designing High-Frequency DC-to-DC Converters With the Si9114A Switchmode Controller

Vishay Semiconductors 

AN701

Vishay Siliconix

FUNCTIONAL DESCRIPTION OF THE

SI9114A BID/CMOS PWM CONTROLLER

The Si9114A controller is similar in configuration to the Si9110.

It uses a traditional constant frequency current mode control,

the most commonly used architecture. The duty cycle is limited

to less than 50% to avoid problems with core reset.

Current mode control is presently the de-facto standard for

PWM control circuits. Indeed, it is the only candidate that

should be considered, given its many advantages:

S Cycle by cycle current limit protection

S Simple loop compensation, eliminating effect of output

inductor

S Excellent fast transient response due to inner control loop

S Automatic input voltage feed-forward compensation

PIN 1 Ć HIGH VOLTAGE PREĆREGULATOR

All switchmode power supplies face a start-up problem caused

by the large difference between dc bus voltage and the VCC

power rail for supplying the control circuit. The traditional

technique has been to keep the control circuit in “sleep mode,”

while a small amount of energy is used to “top up” a large

enough electrolytic capacitor to get the circuit started. When

the circuit starts operating, a winding on the transformer is then

used to power the control circuit. Disadvantages with this type

of circuit include delayed start-up and large required

capacitances for guaranteed operation over the full voltage

range. The Si9114A overcomes these problems by using low

power consumption, BiC/DMOS circuitry, and a unique

high-voltage depletion mode MOSFET. (See 2)

When power is first applied, the depletion transistor is on, and

current flows from the input capacitor CIN into the VCC

capacitor CVCC until VCC reaches 9.2 V. The converter

transformer will then supply the VCC through a bias winding,

which will raise VCC to a level higher than 9.2 V. Ideally this will

be between 11 and 13 V, thus turning off the high-voltage

+VIN

depletion mode MOSFET. The 9.2-V threshold has a

hysteresis of 300 mV to prevent oscillations when the

transition voltage is not clearly defined or when high-line

supply impedance is encountered.

For applications where the input dc voltage is not high, and the

chip power consumption is not excessive, the feedback

winding can be eliminated. In such cases, the pre-regulator

circuit will behave just like a linear regulator with 9.2-V output

and 10-kW series resistance. In this case, the parameters to be

considered are the dropout voltage at lowest line condition and

the power dissipation at highest voltage. The high-voltage

depletion mode MOSFET contains an internal body diode, and

in situations where the VCC is being powered from a laboratory

supply, care must be taken to avoid loading the +VIN rail

beyond the current rating of this device. Typically, the reverse

characteristics of the device will generate a voltage of 3.4 V on

Pin 1 with 10-kW load when powering VCC from a lab supply.

In some applications it is necessary to inhibit the start of a

converter until a high enough voltage is present on the supply

bus. This is the case for the following reasons:

S Circuitry fed from a high line impedance such as a

telephone line will have difficulty starting, since the

converter will behave like a negative impedance. As the dc

voltage decreases, the input current increases because

constant power is drawn. This causes severe oscillations,

and can in some instances have a destructive effect on the

converter. [4]

S During start-up, the Si9114A will begin operation as soon

as the UVLO threshold is reached. Since the converter is

designed to operate over a much higher range—for

example, from 36 to 72 V—then between 10- and 36-V

input the output voltage will be out of regulation and

undefined. In some cases, digital circuitry will not accept

this mode of operation, and system faults will be

encountered without a RESET watchdog circuit.

To overcome these problems, a Zener diode of suitable value

VZ can be placed in series with the +Vin pin, preventing start-up

until VZ + 9.2 V is reached.

To Internal Circuits

VCC

Transformer Winding

CIN

–VIN

www.vishay.com

2

–

+

0.2 V

9.2 V

300 mV

–

UVLO

+

CVCC

Figure 2 Start Circuit

Document Number: 70575

16-Jan-01

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