9015 Datasheet, PDF (3/7 Pages) List of Unclassifed Manufacturers – QUAD NOR GATE

A RCD Type Clamp Circuit
Usually referred to as the RCD type (Resistor, Capacitor, and Diode), these clamp circuits are
used to reset the main transformer to protect from potential saturation problems in this applica-
tion. In Figure 1, the capacitor charges to the level of voltage of the diode to reset the primary
transformer. Then the resistor dissipates the energy stored in the reservoir capacitor. These
components develop varying clamp voltages by adjusting the value of the respective compo-
nents. A high clamp voltage with an amplitude greater than twice the input voltage can deal
with a maximum duty cycle beyond 50%. Because of this, the RCD type is particularly useful in
a wide range of input supply designs. Clamp circuits have their advantages and disadvan-
tages. The elimination of the reset winding is an obvious advantage in the reduction of trans-
former cost. The second advantage is that it is useful in a wide range of input power supplies.
The main disadvantage of an RCD clamp circuit is the power loss from the resistor, resulting in
the decrease of efficiency to some degree. The ground action of this simple clamp circuit is to
reset the transformer at the end of the off period regardless of the level of the input voltage by
adjusting the voltage across the capacitor with various values of three components. The sim-
ple process of acquiring an RCD network is as follows:
The energy stored in the primary transformer per one switching cycle is:
P_ep = ¹²--- × L_mag × I_{m2 ag(pk)} + ¹²--- × L_s × I_{L2 (pk)} × f
L_mag: the transformer magnetizing inductance
I_mag(pk): the peak magnetizing current
L_s: the leakage inductance of the primary transformer
I_L(pk): the peak current in the leakage inductance
f: the operating frequency
If the resistor dissipates the above energy, the main transformer is reset.
V_{---R--R2-- = Pep}
V_R: the voltage across the resistor
These RCD type designs often need repetitive work with much of the development effort spent
on designing the proper clamp network under all operating conditions. They are less likely to
be at optimal efficiency at all lines and loads. Nevertheless, the RCD clamp circuit is very pop-
ular because it can make a system fairly efficient and cost-effective.
Designing the Transformer
After reviewing the core information offered by the various core manufacturers, it is obvious
that the EER 3542 core adequately fits this application. The full height and width of the bobbin
are needed for the required number of turns.
The following equations calculate the minimum number of turns needed by the primary trans-
former at the lowest DC bus voltage, which is 200V in this case, and the voltage per turn (V_pt):
V_P(min) = ^V-----ⁱ--ⁿ---⁽--^m----_∆ⁱ--ⁿ--_B-⁾---^×--_×---^T--_A---^o--_e-ⁿ---⁽--^m-----^a--^x----⁾ = ₀----_.--₂-²---_T-⁰---⁰--_×--^V---₁----^×₀----₉₅----_m^µ----^s-_m------₂-
= 45 turns, approx. 50 turns
V_pt = ^V-----ⁱ--ⁿ--_N-⁽--^m----ⁱ--ⁿ---⁾ = ²--₅--⁰--₀--⁰--_T--^V--- = 4V/turn
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Rev. B, July 2000