APPLICATION NOTE LATERAL MOSFET DESIGN RECOMMENDATIONS FOR AUDIO AMPLIFIERS Semelab Coventry Road Lutterworth Leicestershire LE17 4JB Telephone +44 (0) 1455 554711 Fax +44 (0) 1455 558843 Email: [email protected] Website: http://www.semelab-tt.com 222 Introduction Semelab’s range of lateral mosfets has been specifically designed for audio power amplifiers and there are a number of advantages that they exhibit over bipolar transistors in these applications. This application note is intended to highlight these differences and give design guidelines in using the mosfets for high power ` audio products. Semelab’s new range of Alfet devices are available in both single (8Amp) and double-die Figure 1 – Complimentary N & P channel device (16Amp) versions with complementary N- channel and P-channel parts rated at both 160V Lateral Mosfet AdvaAdvannnntagestages and 200V. Table 1 outlines the currently The key advantages of lateral mosfets over available parts. bipolar junction transistors (BJTs) and vertical mosfets are: Table 111 • Complete absence of secondary breakdown Part Number Pol Voltage Current Package when compared to BJTs. This results in very ALF08N16V/ALF08P16V N/P 160V 8A TO-247 rugged performance and simplified ALF08N16K/ALF08P16K N/P 160V 8A TO-3 protection. ALF08N20V/ALF08P20V N/P 200V 8A TO-247 • Self limiting current characteristic. No ALF08N20K/ALF08P20K N/P 200V 8A TO-3 matter how much drive is given to a lateral ALF16N16W/ALF16P16W N/P 160V 16A TO-264 mosfets they will get to a point where they will not deliver further current. This factor, ALF16N16K/ALF16P16K N/P 160V 16A TO-3 contributed with the previous advantage, ALF16N20W/ALF16P20W N/P 200V 16A TO-264 gives the devices their bullet-proof ALF16N20K/ALF16P20K N/P 200V 16A TO-3 reliability. ALF08NP16V5 N&P 160V 8A TO-247-5 • In rare cases that a device does fail the ALF08NP20V5 N&P 200V 8A TO-247-5 resulting damage in the amplifier is usually limited to the output stage and does not Double-die devices incorporate 2 parallel tend to cause the typical chain reaction connected die that are taken from adjacent back through the amplifier that is often positions on the silicon wafer and this seen in bipolar designs. • guarantees a very high degree of matching of Simple and stable bias because of the low characteristics such that the 2 pieces of silicon current point where the temperature behave as a single device with twice the current characteristics cross from positive to capability. Other highlights in this range include negative coefficient. This is an advantage a new N and P-channel complementary part in a when compared to both vertical mosfets 5-pin power package that has a symmetrical and bipolars that many people are not fully pinout. This device allows a 100W-150W aware of! • amplifier to be achieved with a single output No charge storage effects like bipolar device resulting in a very small footprint that can transistors meaning that they are capable of prove beneficial in applications requiring multi- higher frequency operation and do not channel output in a small box. Figure 1 shows exhibit crossover distortion switching at this new 5-pin version of the TO-247 package. higher frequencies. • No beta droop characteristic of gain at As can be seen in the associated internal higher currents that BJTs exhibit. This diagram, the pinout is totally symmetrical contributes to consistent distortion allowing an optimised pcb layout to be performance across wide ranges of load achieved. impedance. • Easy to drive compared to BJTs due to the high impedance gate. This not only simplifies the drive stages of the amplifier that are required (smaller devices and lower cost) but also results in the voltage amplifier stage generating less distortion. 333 • Integral anti-parallel protection diode. The distortion (most readily observed with a 20kHz intrinsic mosfet parallel body diode means audio signal into load at high amplitude) and that an external diode does not need to be therefore the designer needs to make tradeoffs added as in BJT amplifiers. based on the requirements of the design. This application note will look at some of these This is one of the key characteristics that make advantages in a lot mor e detail as well as lateral mosfets so well suited to class -AB audio outlining some good practices to use when amplifier applications. Stable bias against designer audio power amplifiers with lateral temperature results in a number of significant mosfets. advantages: Biasing Considerations • Complete absence of thermal runaway risk It is commonly known and quoted that bipolar • Simple bias circuitry . An adjustable or fixed transistors have a positive temperature value resistor is adequate and no thermal coefficient (ptc) and that mosf ets have a coupling between bias devices and the negative temperature coefficient (ntc) in their heatsink is required . Transistor based diode relationship of drain current to temperature. multipliers with thermal coupling are not Whilst this ntc characteristic of mosfets is needed. certainly true, it does, however, only occur • No requirement for source resistors for above a particular operating drain current. For single device or parallel device operation lateral mosfets from Semelab this current level is (more on this in a later section) around 100mA , which it just so happens is the • Devices cool faster and return to a stable ideal point for optimum biasing of a cl ass-AB temperature faster after high power use audio amplifier! Figure 2 shows a typical • Much less critical bias point setting than transfer characteristic for the ALF 08N20V and bipolar amplifiers that does not drift with demonstrates a thermal coefficient crossover temperature allo wing easier setup and point at around 110mA. At this point the consistency in production thermal coefficient is in fact zero and therefore completely stable with fluctuations in There are other mosfets on the market aimed at temperature as the output stage warms up at audio applications that are not lateral types but turn on or is hot after high pow er usage. instead of a vertical structure. These do not exhibit the same thermal characteristics and a Figure 2 - ALF16N20W Transfer transfer curve fo r a typical device in shown in Characteristic Figure 3. 1 0.9 0.8 (A) (A) (A) (A) 0.7 D D D D 0.6 0.5 0.4 0.3 0.2 DrainDrain Current Current I I DrainDrain Current Current I I 0.1 0 0 0.5 1 1.5 2 Gate Source Voltage V (V) Gate Source Voltage V GSGSGS (V)(V) Figure 2 – ALF08N20 Transfer Characteristics around temperature coefficient crossover point Figure 3 – Transfer characteristic of typical vertical Although this is the optimum bias point for mosfet stable thermal operation some high power designs that need to minimise the power As can be seen from this curve, the temperature dissipated under quiescen t bias conditions often crossover point does not occur until the drain run the devices at a lower current. Although current is at around 8A! Therefore, for there will be a small shift of the operating point operation at bias levels of 100mA, source with temperature it is still quite small and resistors and thermally coupled transistor bias acceptable and 50mA will work well. Taking the circuits like those used for bi polar amplifiers are bias too low will eventually result in cross over necessary for stable thermal operation to control 444 runaway and for paralleled applications. The Clipping Mechanism in Mosfet Switching mosfets also used in some amplifier Amplifiers designs also exhibit similar characteristics (with The output voltage swing capability and thermal crossovers often at even higher resulting clip point is a result of different currents) and similar steps in the design to parameters than the V induced clipping that provide stable thermal operation are also ce(sat) required. As a result lateral mosfets amplifiers occurs in bipolar power amplifiers. There are 2 are far simpler to set-up with reduced potentially contributing factors with a mosfet: • component requirements for stable bias Rds(on) induced clipping. When the device is operation that results in a smaller footprint and fully turned on it behaves as a resistor with the eradication of these source resistors. an approximate value of 1 Ω per die. Simplified bias circuitry and reduced pcb area Therefore 6A of current per die would mean mean a cost reduction in both materials and that it exhibits a saturation voltage of 6V labour that should be considered in the total and the amp would only swing within 6V of solution cost comparison. This exceptional the power supply rail. thermal stability also enhances reliability. • Gate voltage (Vgs) induced clipping. To be Figure 4 shows a typical lateral mosfet output able to deliver a give drain current the stage and the recommended bias arrangement. mosfet will need a corresponding value of The values of RBF and RBA are dependent on gate-source voltage. For the 6A example, the rest of the design and the current that is approximately 5V would be required for a present in the preceding stage. O single die device at 25 C (see figure 5) . Figure 5 – ALFET transconductance characteristic Which of these mechanisms happens first – Figure 4 Typical Lateral Mosfet Output Stage depends on a number of factors in the overall design of the amplifier circuit. In the typical As a guideline to the nominal value of this output stage shown in figure 6 the driver stage resistance the following equation can be used: can swing to within about 1V of the rail (the driver stage bias current plus driver transistor saturation). This means that the voltage ͐$. ʚ/*/' ʛ ͌ ƍ ͌ Ɣ ʚΩʛ resulting from the R ds(on) and the voltage required ̓.*0- to drive the mosfet are around the same so the clip point would be about 6V from the power Where: Vbias(total) = the required bias voltage of the N & P channel mosfets added together rail.