DN82 Start up switches for switch mode power supplies Andy Aspinall, Systems Engineer, Zetex Semiconductors The use of Zetex medium voltage MOSFETS for switch mode power supplies High voltage start up Many topologies of switching power supplies power the control circuits from an auxiliary tap on the main switching transformer or coil. This method avoids the need for costly house keeping supplies. However the adoption of this scheme presents the designer with the challenge of creating the initial voltage on the control circuit at start up. Any continuous delivery of current from a high voltage line will cause unwanted dissipation and loss of efficiency. Such a scheme may even take a system out of operational specification for low energy standby applications and may even degrade reliability figures. The main requirement for the design of this start up supply is to provide a high enough current to raise the control IC supply and additionally charge any associated smoothing capacitance. At a pre determined voltage the controller will initiate the first few pulses into the main power supply switch transistor. Once this occurs the supply will become self sustaining and the start up path can be switched off. With start up currents of the order of tens of milliamps and high voltages across the device power dissipations can be high for many milliseconds while start up occurs. For AC line operated systems the typical requirement is 450V VDs devices and for 48V DC-DC systems 100 to 200V is usually required. D3 High voltage Output C2 R1 Q1 VGS D2 VGS D1 C1 VCC Contol IC Q2 Figure 1 Issue 1 - September 2006 © Zetex Semiconductors plc 2006 High voltage MOSFET start up circuit 1 www.zetex.com DN82 Operation Figure 1 shows the MOSFET start up circuit where the MOSFET is biased on by a low current in R1 to form a resistive pass element. C1 charges up to the point where the control IC becomes active, the maximum value of VCC in this phase are limited by Zener voltage D1 minus VGS. Once the control IC becomes active drive pulses are issued to Q2 to form normal switching action in the transformer and power is now fed to the control IC via D2. It is therefore important for the designer to chose a value for D1 so that the supply via D2 is high is greater than D1 minus VGS thus biasing off Q1 in normal operation. D4 High voltage Output C2 R2 R1 Q1 Q3 D2 D3 Vcc C3 D1 Figure 2 C1 Contol IC Q2 Improved high voltage MOSFET start up circuit In addition to the basic requirements fault protection may be added such that if the control IC supply can not be raised by the start up circuit a time out occurs to prevent excessive dissipation in the start up MOSFET. By using an additional transistor a simple circuit could be constructed as shown in Figure 2. Figure 2 shows the modified circuit with a time out function to prevent Q1 remaining on and over heating if the power supply can not start. Q3 is simply turned on after a delay from the time constant of R2 and C3. This circuit provides additional protection under all operating conditions and allows the use of smaller devices than if constant on time of Q1 in a fault mode had to be designed for. Zetex manufactures a range of medium voltage MOSFETS and bipolar transistors suitable for use in this application contained in small outline packages with some offering additional pin spacing for creep age distance compliancy. www.zetex.com 2 Issue 1 - September 2006 © Zetex Semiconductors plc 2006 DN82 Table 1 MOSFETS suitable for start up circuits Device Applications Type Package VDS (V) ZXMN0545G4 Universal N-channel SOT223 450 140 0.60 2 ZVN2120G Lower line voltage N-channel SOT223 200 320 2.00 2 ZXMN10A07Z Telecoms N-channel SOT89 100 1500 4.20 1.5 ZXMN10A07F Telecoms N-channel SOT23 100 1500 3.50 0.625 ZXMN10A08E6 Telecoms N-channel SOT23-6 100 2500 8.6 1.7 Dual Nchannel DN8 100 2600 9.00 1.25 ZXMN10A08DN8 Telecoms ID (mA) IDPulse(A) P (W)(*) D NOTES: (*) Based on 25 x 25mm FR4 S Gap D N/C Gap 2x Gap S S Gap Gap D D G D D G G SOT223 (ZXMN0545G4) SOT89 SOT223 (ZVN2120G) Gap Gap S D G SOT23 D1 G1 D1 Gap S2 D2 S G2 D2 D D D G SOT23-6 Figure 3 Issue 1 - September 2006 © Zetex Semiconductors plc 2006 S1 D SO8 MOSFETS suitable for start up circuits 3 www.zetex.com DN82 The use of Zetex medium voltage bipolar transistors in switch mode power supplies D4 High voltage R1 Output C2 Q1 D2 D3 Vcc D1 Contol IC C1 Q2 Figure 4 High voltage bipolar start up circuit Operation Figure 4 shows the Zetex bipolar start up circuit where the bipolar Q1 is biased on by a low current in R1 to form a resistive pass element. C1 charges up to the point where the control IC becomes active. The maximum value of VCC in this phase is limited by Zener voltage D1 minus VBE of Q1 and the VF of D2. Once the control IC becomes active drive pulses are issued to Q2 to form normal switching action in the transformer and power is now fed to the control IC via D3. It is therefore important for the designer to chose a value for D1 so that the supply via D2 is high is greater than D1 minus VBE Q1 thus biasing off Q1 in normal operation. D4 High voltage C2 Q1 R1 R2 Output Q3 D2 D3 Vcc C3 D1 C1 Contol IC Q2 Figure 5 www.zetex.com Improved high voltage bipolar start up circuit 4 Issue 1 - September 2006 © Zetex Semiconductors plc 2006 DN82 In addition to the basic requirements fault protection may be added such that if the control IC supply can not be raised by the start up circuit a time out occurs to prevent excessive dissipation in the start up bipolar. By using a small outline Zetex transistor a simple circuit could be constructed as shown in Figure 5. Figure 5 shows the same circuits with a time out function to prevent Q1 remaining on and over heating if the power supply can not start. Q3 is simply turned on after a delay from the time constant of R2 and C3. This circuit provides additional protection under all operating conditions and allows the use of smaller devices than if constant on time of Q1 had to be designed for. Zetex manufactures a range of medium voltage bipolar transistors suitable for use in this application contained in small outline packages. Table 2 Device Examples of bipolar transistors suitable for start up circuits Applications Type FMMT459 Telecoms FZT493 Bipolar Line voltage Bipolar Package VCEO (V) IC (mA) SOT23 500 150 50 0.625 SOT223 100 1000 2000 2 ICPulse(mA) P (W)(*) D NOTES: (*) Based on 25 x 25mm FR4 board Issue 1 - September 2006 © Zetex Semiconductors plc 2006 5 www.zetex.com DN82 Notes: Europe Americas Asia Pacific Corporate Headquarters Zetex GmbH Kustermann-park Balanstraße 59 D-81541 München Germany Telefon: (49) 89 45 49 49 0 Fax: (49) 89 45 49 49 49 [email protected] Zetex Inc 700 Veterans Memorial Highway Hauppauge, NY 11788 USA Zetex (Asia Ltd) 3701-04 Metroplaza Tower 1 Hing Fong Road, Kwai Fong Hong Kong Zetex Semiconductors plc Zetex Technology Park, Chadderton Oldham, OL9 9LL United Kingdom Telephone: (1) 631 360 2222 Fax: (1) 631 360 8222 [email protected] Telephone: (852) 26100 611 Fax: (852) 24250 494 [email protected] Telephone: (44) 161 622 4444 Fax: (44) 161 622 4446 [email protected] For international sales offices visit www.zetex.com/offices Zetex products are distributed worldwide. For details, see www.zetex.com/salesnetwork This publication is issued to provide outline information only which (unless agreed by the company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contact or be regarded as a representation relating to the products or services concerned. The company reserves the right to alter without notice the specification, design, price or conditions of supply of any product or service. www.zetex.com 6 Issue 1 - September 2006 © Zetex Semiconductors plc 2006