The invention relates to a switching power supply.
Switching power supplies using discrete components are usually constructed with at least two transistors. One transistor is used for switching current in a primary winding of a transformer coupled in series with a main current path of the transistor. The other one transistor is used for providing over-current protection in the first transistor. It may be desirable to form a switching power supply with a power switching Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) as an active element such that, without any additional active element, both self-oscillation and over-current protection are provided by the MOSFET.
In carrying out an inventive feature, a transformer that is included in a positive feedback path of the MOSFET has a tapped auto-transformer winding. A source terminal of the MOSFET is coupled via a current limiting or sampling resistor to a junction terminal between first and second windings of the tapped auto-transformer winding. The first winding forms the primary winding of the transformer and the second winding is coupled to a gate terminal of the MOSFET to form a regenerative feedback path.
In carrying out another inventive feature, the second winding is direct-current (DC) coupled to the gate terminal to avoid the need for any discrete capacitor in the positive feedback path. Thereby, advantageously, the power supply is simplified.
A power supply, embodying an aspect of the invention, includes a source of input supply voltage. A transformer is coupled to a load for energizing the load. A switching, power metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) has a source terminal coupled to the transformer and has a drain terminal coupled to the input supply voltage. A ramping current that stores magnetic energy in the transformer is produced. The ramping current develops a voltage at the source terminal of the MOSFET. The source terminal voltage is coupled to a gate terminal of the MOSFET via the transformer by an auto-transformer action in a regenerative feedback manner, during a first portion of a switching cycle of the MOSFET, when the MOSFET is conductive. A current sampling resistor is coupled in a current path of the ramping current for developing a ramping, degenerative voltage in the resistor voltage to reduce the conductivity of the MOSFET in accordance with the resistor voltage until a turn off threshold voltage of the MOSFET is reached. When the turn off threshold voltage of the MOSFET is reached, the source terminal voltage changes in a degenerative feedback manner. The change in the source terminal voltage continues until the MOSFET becomes non-conductive at an end of the cycle portion. The MOSFET remains non-conductive until an oscillatory resonant voltage produced from the stored magnetic energy renders the MOSFET conductive again, at an end of a following, second portion of the switching cycle.
a, 2b, 2c and 2d provide corresponding waveforms in the arrangement of
A main current path of a drain current I1 includes drain terminal 101, source terminal 102 and current limiting resistor R2. Current I1 flows when MOSFET Q1 is conductive for storing magnetic energy in transformer Tr1. When MOSFET Q1 is conductive, a source voltage Vs is coupled via resistor R2 to terminal 103 for producing a voltage V(n2) in winding n2 by an auto-transformer action of windings n1 and n2.
In carrying out an inventive feature, source terminal 102 is direct-current (DC) coupled to a gate terminal 105 of MOSFET Q1 via winding n2 and a zener diode D1. Zener diode D1 operates in a breakdown mode to provide DC voltage level shifting. Thereby, advantageously, there is no need for a discrete coupling capacitor in a signal path between terminals 102 and 105.
a, 2b, 2c and 2d provide corresponding waveforms useful for explaining power supply 100 of
At a time T1 of a switching cycle time T of
Current sampling resistor R2 of
In carrying out another inventive feature, the decrease in voltage Vs causes a corresponding decrease in voltage V(n2) in winding n2 of
As a result of current I1 ceasing to flow in winding n1, the stored magnetic energy in transformer Tr1 of
MOSFET Q1 remains non-conductive, during a following interval, T3-T5, of cycle time T of
At start-up, a start-up resistor R1 of
A series arrangement that includes a resistor R3, a reference voltage producing zener diode D3 and a light emitting element of an opto-coupler U1 that is coupled to voltage Vout provides a secondary side regulation. Diode D3 starts conducting when voltage Vout is above a threshold voltage of 6V. Consequently, opto-coupler U1 clamps voltage Vgs such that MOSFET Q1 is prevented from being turned on until voltage Vout is reduced to a level below 6V. The efficiency is 60% and thus within the range of other power supplies with an output power of 1 Watt or less.
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/IB2007/003664 | 11/28/2007 | WO | 00 | 5/21/2010 |