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EHT Regulator Output Stage

IP.com Disclosure Number: IPCOM000047945D
Original Publication Date: 1983-Dec-01
Included in the Prior Art Database: 2005-Feb-08
Document File: 3 page(s) / 51K

Publishing Venue

IBM

Related People

Morrish, AJ: AUTHOR

Abstract

In modern displays, regulated EHT (extra high tension) supplies are needed to reduce the supply impedance for improved 'front of screen' performance. Pulse-width modulator circuits have been used to control the peak primary current in the flyback transformer (FBT) at the end of each period. This in turn controls the energy transferred to the secondary which determines the secondary voltage. Recent investigations have concentrated on using a series switching regulator to control the voltage applied to the FBT. A second high voltage switching transistor switches off at the end of each period causing a flyback pulse to be generated, the voltage of which is determined by the primary current immediately before it switches. Fig. 1 shows the essential elements of a new type of regulation circuit.

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EHT Regulator Output Stage

In modern displays, regulated EHT (extra high tension) supplies are needed to reduce the supply impedance for improved 'front of screen' performance. Pulse-width modulator circuits have been used to control the peak primary current in the flyback transformer (FBT) at the end of each period. This in turn controls the energy transferred to the secondary which determines the secondary voltage. Recent investigations have concentrated on using a series switching regulator to control the voltage applied to the FBT. A second high voltage switching transistor switches off at the end of each period causing a flyback pulse to be generated, the voltage of which is determined by the primary current immediately before it switches.

Fig. 1 shows the essential elements of a new type of regulation circuit. Consider the circuit at the start of the period at some time t=0.; Q1 and Q2, the two switching transistors are both turned on at this time so that the full supply rail voltage, Vcc, is presented across the FBT primary. This produces a current which rises linearly. At some time, t1, determined by the regulator circuit, Q1 is rapidly turned off (see Fig. 2). The current, which has reached a value I1, stops flowing through Q1 and is replaced by removing charge from C1. This capacitor prevents an excessive power dissipation in Q1 and is only of small value. Assuming C1 is small, the voltage across it falls rapidly to ground where diode D1 starts to conduct and prevents the voltage dropping below ground. The voltage across the FBT primary is now almost zero, being the voltage drop across the diodes D1 and D2, the transistor Vce of Q2 and the resistive loss of the FBT. Thus the current remains virtually constant from time t1 until t2. At time t2 the high voltage transistor Q2, turns off. The energy stored as flux in the FBT causes current to keep flowing into the flyback capacitor, C2, charging it up. This creates a high voltage which is transformed up by the FBT. The voltage continues to rise until the secondary diode D3 conducts. The current now flows from the secondary into the secondary capacitance Cs whilst the primary current falls to zero. The remainder of the energy stored in the FBT is now transferred to the secondary. This limits the peak voltage of the flyback pulse generated across C2, giving it a flat top at a value of Vfb. It may be seen that for stable regulation:
L.I1 /2 = C2.Vfb /2 + P.T

where L = primary inductance

P = average output power

T = period When the remaining energy in the FBT has been transferred to the secondary capacitor, the secondary output current falls to zero. Now the voltage stored on the flyback capacitor C2 causes current to flow in the opposite direction in the FBT primary. This charges up C1 to Vcc where a clamp diode D4 starts to conduct, limiting the voltage at this point. The voltage on C2 drops sinusoidally to zero, building up a maximum reverse current in the FBT primary. This c...