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Synchronous Rectification for Switched Set Mode Control of Magamps

IP.com Disclosure Number: IPCOM000016852D
Publication Date: 2003-Jul-21
Document File: 5 page(s) / 2M

Publishing Venue

The IP.com Prior Art Database

Abstract

The concept of the switched set mode control of a magnetic amplifier (magamp) used to regulate the output of a power supply is described in US Patent 6,449,175. This paper describes how this concept can be applied in circuits where synchronous rectification is also used.

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Synchronous Rectification for Switched Set Mode Control of Magamps

I.                    Abstract

The concept of the switched set mode control of a magnetic amplifier (magamp) used to regulate the output of a power supply is described in US Patent 6,449,175.� This paper describes how this concept can be applied in circuits where synchronous rectification is also used.

II.                 Introduction

The basic circuit of the switched set control magamp is shown in figure 1

Figure1

In synchronous rectification, a FET is used in parallel, or in place of the schottky rectifiers. The Rds,on of the FET should be low enough such that the conduction loss of the FET ( Irms2*Rds,on) is less than the conduction loss of the corresponding schottky rectifier ( Vf*Irms ). An example of this as used in a buck regulator is shown in figure 2


Figure 2

The switched set control magamp with synchronous rectification proposed here is cost effective, without sacrificing efficiency. The circuit is shown in figure 3.


Figure 3

As seen from the circuit, the error voltage from the magamp control loop is compared with a ramping waveform to generate gate drive pulses for the syncfet.

III.               Circuit Operation


Refer to figure 4 for the circuit waveforms.

Figure 4

The syncfet needs to be on during the freewheeling portion of the period. That is, from the time the secondary voltage goes to negative to the time when the magamp saturates. Comparing the error voltage of the magamp control loop with a ramping waveform (vramp2) derives the gating signal for the syncfet. Notice that vramp and vramp2 are similar waveforms, they are only 180o out of phase. The waveform vramp when compared to the error voltage generates the gating signal for the secondary switch of the magamp. When this secondary switch turns on, the state of the core...