SC1933C datasheet(18/28 Pages) POWERINT

18 / 28 page

Rev. C 11/18

SC1933C/SC1936C

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Figure 17. Discontinuous Conduction Mode Current Waveform, KP > 1.

Maximum Operating Flux Density, B

M (Gauss)

A maximum value of 3800 gauss at the peak device current limit

(at 132 kHz) is recommended to limit the peak flux density at start-up

and under output short-circuit conditions. Under these conditions the

output voltage is low and little reset of the transformer occurs during

the switch off-time. This allows the transformer flux density to

staircase beyond the normal operating level. A value of 3800 gauss

at the peak current limit of the selected device together with the

built-in protection features of SC1933C/SC1936C IC provide sufficient

margin to prevent core saturation under start-up or output short-

circuit conditions.

Transformer Primary Inductance, (LP)

Once the lowest operating input voltage, switching frequency at full

load, and required VOR are determined, the transformers primary

inductance can be calculated. The PIXls design spreadsheet can be

used to assist in designing the transformer.

Quick Design Checklist

As with any power supply, the operation of all SC1933C/SC1936C

designs should be verified on the bench to make sure that component

limits are not exceeded under worst-case conditions.

As a minimum, the following tests are strongly recommended:

1. Maximum Drain Voltage – Verify that V

DS of SC1933C/SC1936C do

not exceed >650 V of maximum continuous drain voltage at the

highest input voltage and peak (overload) output power in normal

operation and during start-up. Also make sure that SR FET do

not exceed >90% of breakdown voltage under above mentioned

conditions.

2. Maximum Drain Current – At maximum ambient temperature,

maximum input voltage and peak output (overload) power.

Review drain current waveforms for any signs of transformer

saturation or excessive leading-edge current spikes at start-up.

Repeat tests under steady-state conditions and verify that the

leading edge current spike is below I

LIMIT(MIN) at the end of tLEB(MIN).

Under all conditions, the maximum drain current for the primary

switch should be below the specified absolute maximum ratings.

3. Thermal Check – At specified maximum output power, minimum

input voltage and maximum ambient temperature, verify that

temperature specification limits for SC1933C/SC1936C IC,

transformer, output SR FET, and output capacitors are not

exceeded. Enough thermal margin should be allowed for

part-to-part variation of the R

DS(ON) of the SC1933C/SC1936C IC.

Under low-line, maximum power, a maximum SC1933C/SC1936C

SOURCE pin temperature of 110 °C is recommended to allow for

these variations.

KP ≡ KDP =

T = 1/fS

(1-D) × T

(1-D) × T = t

D × T

(b) Borderline Discontinuous/Continuous, KP = 1

(a) Discontinuous, KP > 1

Primary

Secondary

Primary

Secondary

PI-2578-103114

(1-D) × T

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