L

(in V/s)

n = 1+

2mc

m1

(no unit)

Leff =

L

(D')

2

Z

c

(in rad/s)

2fs

nD'

#

(

)

LM5000

www.ti.com

SNVS176D – MAY 2004 – REVISED MARCH 2007

(14)

(15)

(16)

(17)

mc

≊ 0.072fs (in A/s)

(18)

(19)

SWITCH VOLTAGE LIMITS

In a flyback regulator, the maximum steady-state voltage appearing at the switch, when it is off, is set by the

(20)

steady-state voltage . Usually, this voltage spike is caused by the transformer leakage inductance and/or the

output rectifier recovery time. To “clamp” the voltage at the switch from exceeding its maximum value, a transient

suppressor in series with a diode is inserted across the transformer primary.

If poor circuit layout techniques are used, negative voltage transients may appear on the Switch pin. Applying a

negative voltage (with respect to the IC's ground) to any monolithic IC pin causes erratic and unpredictable

operation of that IC. This holds true for the LM5000 IC as well. When used in a flyback regulator, the voltage at

the Switch pin can go negative when the switch turns on. The “ringing” voltage at the switch pin is caused by the

output diode capacitance and the transformer leakage inductance forming a resonant circuit at the

secondary(ies). The resonant circuit generates the “ringing” voltage, which gets reflected back through the

transformer to the switch pin. There are two common methods to avoid this problem. One is to add an RC

snubber around the output rectifier(s). The values of the resistor and the capacitor must be chosen so that the

voltage at the Switch pin does not drop below

−0.4V. The resistor may range in value between 10Ω and 1 kΩ,

and the capacitor will vary from 0.001

μF to 0.1 μF. Adding a snubber will (slightly) reduce the efficiency of the

overall circuit.

The other method to reduce or eliminate the “ringing” is to insert a Schottky diode clamp between the SW pin

and the PGND pin. The reverse voltage rating of the diode must be greater than the switch off voltage.

OUTPUT VOLTAGE LIMITATIONS

The maximum output voltage of a boost regulator is the maximum switch voltage minus a diode drop. In a

flyback regulator, the maximum output voltage is determined by the turns ratio, N, and the duty cycle, D, by the

equation:

(21)

The duty cycle of a flyback regulator is determined by the following equation:

(22)

Theoretically, the maximum output voltage can be as large as desired—just keep increasing the turns ratio of the

transformer. However, there exists some physical limitations that prevent the turns ratio, and thus the output

voltage, from increasing to infinity. The physical limitations are capacitances and inductances in the LM5000

switch, the output diode(s), and the transformer—such as reverse recovery time of the output diode (mentioned

above).

Copyright © 2004–2007, Texas Instruments Incorporated

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