Design Note 21 Issue 2 October 1995 Design Note 21 Issue 2 October 1995 Zetex E-Line transistors are optimised to possess a very low VCE(sat), and are therefore much more efficient in high current switching applications than conventional power products. Automotive Alarm Siren Driver 2A DC Rated Super-ß Transistors Provide Reduced Component Count ’H’ Bridge Introduction Background The majority of modern automotive alarm sirens employ a moving coil lou dspe aker , that is driv en by a n ’H’-bridge output stage. This output driver circuit is commonly effected with T O - 1 2 6 / T O- 2 2 0 p a c k a g e d p o w e r transistors, as the currents involved and the resulting power dissipation are both in excess of common smaller package products. Zetex Super E-Line (TO-92 style) transistors however, due to their superior device geometries and silicone packaging, can, and have been used in this application to provide a minimal drive circuitry topology, and a size, cost and performance advantage. The circuit diagrams presented in Figures 1 and 2, show two common methods of effecting a siren driver circuit. These circuits include collector-emitter (C-E) diodes to protect the transistor in the event of negative rail excursions, due to i n d u c ti ve lo ad tr ansients, an d base-emitter (B-E) resistors to provide a path for any transistor leakage current. Th is latter point is an impor tant consideration when using the usual TO-126/TO-220 power products, as their high VCE(sat) can lead to appreciable temperature rises. important differences to note: i) As the devices employ an efficient matrix die geometry, they produce low VCE(sat) values, this leads to very low device dissipation, and therefore low temperature rise. (Viz. ZTX690B: VCE(sat) = 0.3V at IC=2A, IB =20mA and ZTX790A: VCE(sat) = 0.4V at IC=2A, IB =50mA, please consult ZTX688BZTX696B and ZTX788B - ZTX796A series datasheets). This can be very important in automotive applications, where the operating environment can reach high ambient temperatures. It follows that in this case, the usual base-emitter resistors employed to aid turn-off, and to divert leakage currents at high junction temperature, are no longer required. Some siren manufacturers use TO-220 packaged Darlington power transistors. While this can reduce the component count, (though can also increase the component cost), as the C-E diodes are sometimes integral to the Darlington, the intrinsic high on-state voltage of the Darlington configuration increases device dissipation. Consequently the circuit requires B-E resistors, and also reduces the power delivered to the load. Circuit description The circuit shown in Figure 3 has been de vised using Ze tex Super E-Line transistors. It is basically a modified version of figure 2. There are two + ZTX 790A ZTX 790A ZTX 690B ZTX 690B + + ii) The Zetex devices chosen, possess a reasonable hFE, (strictly termed hFC) when used in the inverted mode, Eg. with the collector/emitter terminals reversed. Typical peak values being 125 for the ZTX690B and 100 for the ZTX790A . This feature means that they are still capable of operating when subjected to reverse transients, such as when driving an inductive load, and therefore do not always require protection diodes. For the siren driver application, the C-E diodes may be confidently omitted. O/P O/P O/P _ _ Figure 1. Conventional Siren Driver Circuit using Separate Pre-driver Transistors. _ Figure 2. Conventional Siren Driver Circuit using Cross-coupled Darlington Pre-driver Configuration. DN21-1 Figure 3. ZETEX Reduced Component Count ’H’-Bridge Circuit. DN21-2 Design Note 21 Issue 2 October 1995 Design Note 21 Issue 2 October 1995 Zetex E-Line transistors are optimised to possess a very low VCE(sat), and are therefore much more efficient in high current switching applications than conventional power products. Automotive Alarm Siren Driver 2A DC Rated Super-ß Transistors Provide Reduced Component Count ’H’ Bridge Introduction Background The majority of modern automotive alarm sirens employ a moving coil lou dspe aker , that is driv en by a n ’H’-bridge output stage. This output driver circuit is commonly effected with T O - 1 2 6 / T O- 2 2 0 p a c k a g e d p o w e r transistors, as the currents involved and the resulting power dissipation are both in excess of common smaller package products. Zetex Super E-Line (TO-92 style) transistors however, due to their superior device geometries and silicone packaging, can, and have been used in this application to provide a minimal drive circuitry topology, and a size, cost and performance advantage. The circuit diagrams presented in Figures 1 and 2, show two common methods of effecting a siren driver circuit. These circuits include collector-emitter (C-E) diodes to protect the transistor in the event of negative rail excursions, due to i n d u c tive lo ad tr ansients, an d base-emitter (B-E) resistors to provide a path for any transistor leakage current. Th is latter point is an impor tant consideration when using the usual TO-126/TO-220 power products, as their high VCE(sat) can lead to appreciable temperature rises. important differences to note: i) As the devices employ an efficient matrix die geometry, they produce low VCE(sat) values, this leads to very low device dissipation, and therefore low temperature rise. (Viz. ZTX690B: VCE(sat) = 0.3V at IC=2A, IB =20mA and ZTX790A: VCE(sat) = 0.4V at IC=2A, IB =50mA, please consult ZTX688BZTX696B and ZTX788B - ZTX796A series datasheets). This can be very important in automotive applications, where the operating environment can reach high ambient temperatures. It follows that in this case, the usual base-emitter resistors employed to aid turn-off, and to divert leakage currents at high junction temperature, are no longer required. Some siren manufacturers use TO-220 packaged Darlington power transistors. While this can reduce the component count, (though can also increase the component cost), as the C-E diodes are sometimes integral to the Darlington, the intrinsic high on-state voltage of the Darlington configuration increases device dissipation. Consequently the circuit requires B-E resistors, and also reduces the power delivered to the load. Circuit description The circuit shown in Figure 3 has been de vised using Ze tex Super E-Line transistors. It is basically a modified version of figure 2. There are two + ZTX 790A ZTX 790A ZTX 690B ZTX 690B + + ii) The Zetex devices chosen, possess a reasonable hFE, (strictly termed hFC) when used in the inverted mode, Eg. with the collector/emitter terminals reversed. Typical peak values being 125 for the ZTX690B and 100 for the ZTX790A . This feature means that they are still capable of operating when subjected to reverse transients, such as when driving an inductive load, and therefore do not always require protection diodes. For the siren driver application, the C-E diodes may be confidently omitted. O/P O/P O/P _ _ Figure 1. Conventional Siren Driver Circuit using Separate Pre-driver Transistors. _ Figure 2. Conventional Siren Driver Circuit using Cross-coupled Darlington Pre-driver Configuration. DN21-1 Figure 3. ZETEX Reduced Component Count ’H’-Bridge Circuit. DN21-2 Design Note 21 Issue 2 October 1995 Design Note 21 Issue 2 October 1995 Example Circuit Specification +12V ZTX 790A Q9 ZTX 790A C6 470µ Q10 R7 680 R3 27K R2 27K R1 680 R10 R6 560 47K R5 R4 10K 4K7 R14 330 R13 330 C4 10µ C3 0.1µ Q6 Supply Voltage Range 2 – 25V Output Stage Efficiency 98.5% 6 Ohm moving coil load Average VCE(sat) =0.1V R12 3K3 Vsupply =14.2V C1 0.1u D4 Output C2 10µ Q1 I(dc) =1.2A Q4 D3 ZTX 690B Q8 Q5 ZTX 690B Q2 R9 5K6 D1 R8 5K6 Q3 D2 Q7 R11 3K3 0V 'H' BRIDGE OUTPUT STAGE Figure 4 . Example Circuit (Note 1). Output Stage Efficiency 91.5% 6 Ohm Resistive load Average VCE(sat) =0.55V C5 47µ Vsupply =14.2V I(dc) =2.12A SIGNAL GENERATION CIRCUITRY Q1 - Q6 MPS2222A Q7 - Q8 ZTX690B The modified ’H’- bridge siren driver stage has been incorporated into a complete siren driver circuit, in order to demonstrate the capability of Zetex Super E-Line transistors. The demonstration circuit is shown in figure 4, and consists of two sections; the signal generation circuitry, and the ’H’ bridge output stage. The former comprises a slow running astable multivibrator to generate a ramp signal, that is then used to modulate a second multivibrator, causing it to sweep over a range of frequencies. This section drives the ‘H’-bridge output stage described previously. While the multivibrator method is valid, (some manufacturers use a similar circuit, while others favour op-amp derived circuits, or even small micro-controllers - see Note 2), there can be a problem with cross-conduction, Eg. Simultaneous conduction by both Load dependent; 14.2V nominal Q9 - Q10 ZTX790A D1 - D4 1N4148 NPN and PNP devices during switch-over, thus leading to excessive power dissipation. To prevent this, the output stage on the demonstration circuit has two additional signal diodes, D3 and D4. These components inhibit the driver transistors Q5 and Q6, until the opposing half of the ‘H’-bridge has started to switch off. More elaborate signal generators incorporate delays between changes of output state, (using an extra gate or software) so would not require these extra diodes. The circuit has been designed assuming a load impedance of 6 Ohms; a common value for the drive units used. Lower loads can be driven but may require either a change to the bias resistors, R13 /14, to provide sufficient base current, or for very low impedance loads, a change of output transistors:the ZTX851/ZTX951 with a continuous current rating of 5 Amps and 4 Amps respectively are eminently suitable. DN21-3 Junction Temperature Rise 19°C 6 Ohm moving coil load Control ramp 1 to 9V, 4Hz Vsupply =14.2V Swept frequency range 870Hz – 1.9kHz Vsupply =14.2V Vsupply =14.2V Note 1: D3 and D4 are not required for signal generation circuits possessing output/output deadtime. Note 2: ZETEX now offer an IC solution for the signal generation circuitry - namely the ZSD100. DN21-4 Design Note 21 Issue 2 October 1995 Design Note 21 Issue 2 October 1995 Example Circuit Specification +12V ZTX 790A Q9 ZTX 790A C6 470µ Q10 R7 680 R3 27K R2 27K R1 680 R10 R6 560 47K R5 R4 10K 4K7 R14 330 R13 330 C4 10µ C3 0.1µ Q6 Supply Voltage Range 2 – 25V Output Stage Efficiency 98.5% 6 Ohm moving coil load Average VCE(sat) =0.1V R12 3K3 Vsupply =14.2V C1 0.1u D4 Output C2 10µ Q1 I(dc) =1.2A Q4 D3 ZTX 690B Q8 Q5 ZTX 690B Q2 R9 5K6 D1 R8 5K6 Q3 D2 Q7 R11 3K3 0V 'H' BRIDGE OUTPUT STAGE Figure 4 . Example Circuit (Note 1). Output Stage Efficiency 91.5% 6 Ohm Resistive load Average VCE(sat) =0.55V C5 47µ Vsupply =14.2V I(dc) =2.12A SIGNAL GENERATION CIRCUITRY Q1 - Q6 MPS2222A Q7 - Q8 ZTX690B The modified ’H’- bridge siren driver stage has been incorporated into a complete siren driver circuit, in order to demonstrate the capability of Zetex Super E-Line transistors. The demonstration circuit is shown in figure 4, and consists of two sections; the signal generation circuitry, and the ’H’ bridge output stage. The former comprises a slow running astable multivibrator to generate a ramp signal, that is then used to modulate a second multivibrator, causing it to sweep over a range of frequencies. This section drives the ‘H’-bridge output stage described previously. While the multivibrator method is valid, (some manufacturers use a similar circuit, while others favour op-amp derived circuits, or even small micro-controllers - see Note 2), there can be a problem with cross-conduction, Eg. Simultaneous conduction by both Load dependent; 14.2V nominal Q9 - Q10 ZTX790A D1 - D4 1N4148 NPN and PNP devices during switch-over, thus leading to excessive power dissipation. To prevent this, the output stage on the demonstration circuit has two additional signal diodes, D3 and D4. These components inhibit the driver transistors Q5 and Q6, until the opposing half of the ‘H’-bridge has started to switch off. More elaborate signal generators incorporate delays between changes of output state, (using an extra gate or software) so would not require these extra diodes. The circuit has been designed assuming a load impedance of 6 Ohms; a common value for the drive units used. Lower loads can be driven but may require either a change to the bias resistors, R13 /14, to provide sufficient base current, or for very low impedance loads, a change of output transistors:the ZTX851/ZTX951 with a continuous current rating of 5 Amps and 4 Amps respectively are eminently suitable. DN21-3 Junction Temperature Rise 19°C 6 Ohm moving coil load Control ramp 1 to 9V, 4Hz Vsupply =14.2V Swept frequency range 870Hz – 1.9kHz Vsupply =14.2V Vsupply =14.2V Note 1: D3 and D4 are not required for signal generation circuits possessing output/output deadtime. Note 2: ZETEX now offer an IC solution for the signal generation circuitry - namely the ZSD100. DN21-4