WO1995006350A1

WO1995006350A1 - Thermal shutdown for power converters

Info

Publication number: WO1995006350A1
Application number: PCT/CA1994/000451
Authority: WO
Inventors: Ivan Meszlenyi
Original assignee: Powerpaq Technologies Inc.
Priority date: 1993-08-20
Filing date: 1994-08-19
Publication date: 1995-03-02
Also published as: CA2104568A1; AU7487494A

Abstract

A magnetic thermal shutdown for use in a power converter comprising a pair of switching elements connected in a bridge mode configuration; a control device for controlling the switching operation of the switching elements; a resonant network; a transformer having a low curie temperature lossy magnetic core with a primary winding and a secondary winding, the primary winding being connected to an output capacitor and the secondary winding being connected to a rectifying device and a load; wherein the switching elements and the rectifying device are implemented using power semiconductors; a thermally conductive material surrounding the magnetic core; a common heat sink thermally coupled to all power semiconductors as well as the magnetic core; and a high thermal resistive element to separate the thermal conductive material from the common heat sink.

Description

THERMAL SHUTDOWN FOR POWER CONVERTERS

Field of the Invention

This invention is related in general to switch mode power converters, and more particularly to power mode converters which are protected from thermal runaway by means of thermal shutdown. Brief Description of Drawings

In drawings which illustrate the prior art and embodiments of the invention:

Figure 1 is a block schematic diagram of a conventional thermal shutdown circuit as incorporated into a switch mode power supply;

Figure 2 is a block schematic diagram of a switch mode power supply incorporating thermal shutdown management according to an embodiment of the present invention; and

Figure 3 is a drawing of a transformer core with appropriate thermal shutdown management according to the preferred embodiment of the invention. Description of the Related Art

Figure 1 illustrates a conventional thermal shutdown circuit as incorporated into a switch mode power supply. In this drawing, reference numeral 15 denotes a power source having a positive terminal connected to a control circuit 1, a first switching element 2, and a first snubber capacitor 4. A negative terminal of power source 15 is also connected to control circuit 1, a second switching element 3, and a second snubber capacitor 5. A pair of outputs of control circuit 1 are connected to corresponding control inputs of switching elements 2 and 3, which are connected in a bridge circuit to respective snubber capacitors 4 and 5.

An inductor 6 is connected to the junction of snubber capacitors 4 and 5 and to ground via an output capacitor 7, across which the primary winding 8 of a transformer is connected. The secondary winding 9 is connected to rectifying element 10 which is connected to an inductor 11 and output filter capacitor 12, which in turn is connected in parallel to load 13.

Temperature sensor 16 is connected to a resistor 17 and to one input of a comparator 18, which is configured so as to incorporate a predetermined amount of hysteresis. The output of comparator 18 is connected to a shutdown portion (S/D) of control circuit 1. A reference temperature is established by means of a reference source 19 which is connected to the other input of comparator 18.

The circuit operates as follows to protect the switch mode power supply from thermal runaway:

Temperature sensor 16 detects the temperature of the power semiconductors in the control circuit 1 (i.e. switching elements 2, 3 and rectifying element 10) , and in response conducts current proportional to the detected temperature. This current establishes an error voltage across resistor 17, which is applied to comparator 18. The signal output from the comparator 18 is then applied to the shutdown input (S/D) of the control circuit 1 so that when the detected temperature exceeds the pre-set reference temperature as established by reference source 19, control circuit 1 disables switching elements 2 and 3. Summary of the Invention

It is an object of the present invention to provide a thermal shutdown circuit which enables protection of switch mode power supplies from damage due to overheating, without the use of additional circuitry. Another object of this invention is to provide an economical implementation of such a magnetic thermal shutdown circuit.

These and other objects are achieved by the present invention which is an improvement over the above described prior art approach, and which includes no additional components other than those absolutely required for basic circuit operation of the switch mode power supply, and still provides thermal protection by use of an appropriate curie temperature lossy magnetic core for the transformer and a simple means of thermal management. Description of the Preferred Embodiment

An embodiment of the present invention is described below with reference to Figures 2 and 3 of the attached drawings, in which circuit elements which are common to the prior art circuit of Figure 1 are represented by identical reference numerals.

Figure 2 shows a power source 15 having a positive terminal which is connected to a control circuit 1, a first switching element 2 and first snubber capacitor 4 in the usual manner, and a negative terminal connected to control circuit 1, second switching element 3 and second snubber capacitor 5. The control outputs of circuit 1 drive the first and second switching elements 2, 3 in the usual manner. The junction of switching elements 2, 3 and snubber capacitors 4, 5 drives a resonant tank comprised of inductor 6 and output capacitor 7. The primary winding 8 of the transformer is connected in parallel to the resonant tank. The secondary winding 9 is connected to rectifier 10 which is connected to inductor 11 and output filter capacitor 12, which is connected in parallel to load 13. The control circuit 1 contains all of the conventional components needed to drive switching elements 2 and 3, but does not require a thermal shutdown portion as in the prior art.

The switching elements 2, 3 and rectifying element 10 are implemented using power semiconductors, according to established design. The parallel loaded series resonant network (i.e. resonant tank) is fed by a square wave generated by the switching elements 2 and 3. The transformer primary winding 8 is connected in parallel to resonant capacitor 7, as indicated above, and the transformer output is rectified to produce a resultant DC output voltage. According to an important aspect of the present invention, primary winding 8 and secondary winding 9 are placed on a lossy magnetic core 16 having a relatively low curie temperature as discussed in greater detail below.

Figure 3 shows a common heat sink 19 which is thermally coupled to the power semiconductors 2, 3 and 10, for dissipating heat generated by the semiconductors 2, 3 and 10. The common heat sink 19 is also thermally coupled to the core 16 of the transformer by a high thermal resistive element 18 and a thermally conductive material 17 which envelopes the core in such a way that the thermal differential set up by the power flow of the transformer maintains a safe temperature for the power semiconductors.

In the event of a rise in temperature which results in the curie temperature of core 16 being reached, the core material changes from a ferromagnetic state to a paramagnetic state such that transformer primary 8 immediately presents a very low inductance, thereby essentially short circuiting the capacitor 7 of the resonant tank and halting power flow. When operating in this condition, the only power dissipated by the circuit is that required to cycle the energy through the series resonant inductor 6. Thus, the power semiconductors 2, 3 and 10, and the transformer core 16 are allowed to cool off, and upon lowering of the previously high ambient temperature such that the temperature of core 16 falls below its curie temperature, original operating conditions are recovered.

For a given power dissipation P generated in the core 16, the common heat sink temperature T_b is given by

T_b = T_e-(PR,) (1) wherein T_c denotes the curie temperature of the core 16, and _t denotes the thermal resistance of the isolating element 18.

Temperature differential T_d is given by T_d = T_b - T_c (2)

Since the power P remains relatively constant with all line and load variations in the topology of the embodiment disclosed herein, then temperature differential, T_d also remains relatively constant.

Other embodiments and modifications of the invention are possible. For example, although the preferred embodiment as disclosed herein is implemented in a power transformer, the inventive principle of thermal shutdown according to the present invention may be applied to drive transformers or any other transformers used in a circuit configuration with various power semiconductors wherein the transformer core is characterized by a low curie temperature so that the primary winding provides a low inductance to the circuit when the curie temperature is exceeded, thereby effectively short circuiting the current path and allowing the components to cool.

Claims

I Claim:

1. A transformer-based magnetic shutdown circuit for driving an output circuit, comprising: a) power supply means; b) a pair of switching elements connected in a bridge mode configuration to said power supply means, said pair of switching elements being implemented using power semiconductors; c) control means for controlling switching operation of said switching elements; d) a resonant network connected to a junction of said pair of switching elements, said resonant network including at least an inductor and an output capacitor; e) transformer means having a low curie temperature lossy magnetic core, a primary winding and a secondary winding, wherein the primary winding is connected in parallel to said output capacitor and wherein the secondary winding is connected to said output circuit; f) thermally conductive means surrounding said magnetic core; g) common heat sink means thermally coupled to said power semiconductors and said magnetic core; and h) high thermal resistive element means separating said thermally conductive means from said common heat sink means.

2. The magnetic thermal shutdown circuit according to claim 1, wherein said output circuit comprises a rectifying means also implemented using said power semiconductors, and a load connected in a circuit to said rectifying means.

3. The magnetic thermal shutdown circuit according to claim 2, further including an output inductor and output filter capacitor connected intermediate said rectifying means and said load.

4. The magnetic thermal shutdown circuit according to claim 1, wherein said thermally conductive means surrounds said transformer means.