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AN1621 APPLICATION NOTE

Table 1. Power Supply Specification

The core is E20/10/5-3C85, 10 turns/winding, no air-gap.

A high permeability core minimises the magnetising current to maintain a correct operation far away of the core

saturation point, at maximum duty cycle and high core temperature.

The proposed gate drive circuit allows the use of the 1:1:1 turns ratio drive transformer; moreover, a controlled

drain current rise time (by R15) and a fast fall time are achieved. A very short min. Ton pulse give the possibility

to stabilise the output voltage at max. mains and min. load.

2.4

Power MOS selection

The two-switch topology allows to use the power elements with a voltage breakdown equal to the max. rectified

mains voltage.

The mosfet used here is the STW14NK50; this device, with 500V of BVdss give us also some safety margin.

The basic parameters of the STW14NK50 are listed below:

Rdson (25°C) = 0.38

Ω max., at Id = 7A (0.76Ω max. at 100°C)

Coss = 300pF typ., Qg = 75nC typ., package in TO-247

At min. supply voltage and max. load current, the conduction losses for each transistor are:

former section.

Estimating in about 3.2W switching and parasitic losses, the total power losses of each transistor are about 7W.

Considering 100°C of maximum operating junction temperature (at 40°C of ambient temperature) and a thermal

resistance junction-heatsink of 0.66°C/W, a heatsink of 4°C/W is required to dissipate both the transistors.

2.5

Current sense

Considering the output current rating of 13A continuous, it's our opinion that a current transformer for current

sensing is the best approach for maximising the efficiency, reliability and internal ambient temperature in case

the power supply has to be housed in a plastic box.

Due to the constant current limiting requirements, as shown in Fig2, a couple of current transformers have been

used; one transformer is sensing the current flowing into D1 (in conduction when T1 and T2 are ON) and the

second one is sensing the current flowing into D5, recirculation diode.

Oring the two transformers by D2 and D3, and closing the loop with a proper impedance value, R1, we realise

a voltage signal reproducing exactly the inductor current shape.

The current sensing loop is closed by R1 selected according the transformers turns.

Two small toroid ferrite cores (41005-TC, Magnetics, F material, 3000

µ) have been used, with 50 turns.

R1 is defined by:

where:

1V is the nominal threshold voltage of the current sense.

Ipk is the inductor peak current( considering a 20% of current ripple, Ipk = Io +

∆I/2 = 13 + 1.3 = 14.3A)

Symbol

Description

Parameter

Input voltage

220Vac (176 Vac to 265 Vac) // 50Hz

Output voltage

24V % (ripple voltage <1%)

Output Current

13Amax. continuous, 0.5Amin

Limiting Current

constant type till ground

Output continuos power

312W

Switching Frequency

200kHz

η

Target Efficiency (full load)

= 90% (from mains to output)

R1

50 1V

⋅

lpk

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=