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AN44
A high power LED driver for low voltage halogen replacement
Introduction
LED lighting is becoming more popular as a replacement technology for Halogen low voltage lighting, primarily because of the low efficiency, reliability and lifetime issues associated with Halogen bulbs. Discussed below is a novel approach for driving high power LED's as a replacement for low voltage halogen lighting systems. A typical schematic diagram is shown in Figure 1.
Figure 1 Operation
Schematic diagram
Please refer to the typical schematic diagram in Figure 1. On period, TON The ZXSC300 turns on Q1 until it senses 19mV (nominal) on the ISENSE pin. The current in Q1 to reach this threshold is therefore 19mV/R1, called IPEAK. With Q1 on, the current is drawn from the battery and passes through C1 and LED in parallel. Assume the LED drops a forward voltage VF. The rest of the battery voltage will be dropped across L1 and this voltage, called V(L1) will ramp up the current in L1 at a rate di/dt = V(L1)/L1, di/dt in Amps/sec, V(L1) in volts and L1 in Henries. The voltage drop in Q1 and R1 should be negligible, since Q1 should have a low RDS(on) and R1 always drops less than 19mV, as this is the turn-off threshold for Q1. VIN = VF + V(L1) TON = IPEAK x L1/ V(L1)
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So TON can be calculated, as the voltage across L1 is obtained by subtracting the forward LED voltage drop from VIN. Therefore, if L1 is smaller, TON will be smaller for the same peak current IPEAK and the same battery voltage VIN. Note that, while the inductor current is ramping up to IPEAK, the current is flowing through the LED and so the average current in the LED is the sum of the ramps during the TON ramping up period and the TOFF ramping down period.
Off period, TOFF
The TOFF of ZXSC300 and ZXSC310 is fixed internally at nominally 1.7s. Note that, if relying on this for current ramp calculations, the limits are 1.2s min., 3.2s max. In order to minimize the conductive loss and switching loss, TON should not be much smaller than TOFF. Very high switching frequencies cause high dv/dt and it is recommended that the ZXSC300 and 310 are operated only up to 200 kHz. Given the fixed TOFF of 1.7s, this gives a TON of (5s 1.7s) = 3.3s minimum. However, this is not an absolute limitation and these devices have been operated at 2 or 3 times this frequency, but conversion efficiency can suffer under these conditions. During TOFF, the energy stored in the inductor will be transferred to the LED, with some loss in the Schottky diode. The energy stored in the inductor is: 1/2 x L x IPEAK2 [Joules] Continuous and discontinuous modes (and average LED current) If TOFF is exactly the time required for the current to reach zero, the average current in the LED will be IPEAK/2. In practice, the current might reach zero before TOFF is complete and the average current will be less because part of the cycle is spent with zero LED current. This is called the `discontinuous' operation mode and is shown in Figure 2.
Figure 2
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For continuous mode If the current does not reach zero after 1.7s, but instead falls to a value of IMIN, then the device is said to be in `continuous' mode. The LED current will ramp up and down between IMIN and IPEAK (probably at different di/dt rates) and the average LED current will therefore be the average of IPEAK and IMIN, as shown in Figure 3.
Figure 3
Design example
(Refer to Figure 1 and Table 1) Input = VIN = 12V LED forward drop = VLED = 9.6V VIN = VLED+VL Therefore VL = (12 - 9.6) = 2.4 The peak current = VSENSE / R1 (R1 is RSENSE) = 24mV/50mR = 480mA TON = IPEAK x L1/V(L1) 680mAx22H T ON --------------------------------------- = 6.2s 2.4 These equations make the approximation that the LED forward drop is constant throughout the current ramp. In fact it will increase with current, but they still enable design calculations to be made within the tolerances of the components used in a practical circuit. Also, the difference between VIN and VLED is small compared to either of them, so the 6.2s ramp time will be fairly dependent on these voltages. Note that, for an LED drop of 9.6V and a Schottky drop of 300mV, the time to ramp down from 680mA to zero would be: 680mAx22H TDIS --------------------------------------- = 1.5s ( 9.6 + 0.3 )
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As the TOFF period is nominally 1.7s, the current should have time to reach zero. However, 1.5s is rather close to 1.7s and it is possible that, over component tolerances, the coil current will not reach zero, but this is not a big issue as the remaining current will be small. Note that, because of the peak current measurement and switch-off, it is not possible to get the dangerous `inductor staircasing' which occurs in converters with fixed TON times. The current can never exceed IPEAK, so even if it starts from a finite value (i.e. continuous mode) it will not exceed the IPEAK. The LED current will therefore be approximately the average of 680mA and zero = 340mA (it will not be exactly the average, because there is a 200ns period at zero current, but this is small compared with the IPEAK and component tolerances). Ref U1 Q1 D1 D2 L1 R1 R2 C1 C2 C3 1A / 40V 6V8 22 H 50m 1k2 100 F/25V 1 F/10V 2.2 F/25V Value Part number ZXSC310E5 ZXMN6A07F ZHCS1000 Generic Manufacturer Contact details Zetex Zetex Zetex Generic www.coilcraft.com 0805 size 0805 size www.zetex.com www.zetex.com www.zetex.com Comments LED Driver in SOT23-5 N-channel MOSFET in SOT23 1A Schottky diode in SOT23 6V8 Zener diode
DO3316P-223 Coilcraft Generic Generic Generic Generic Generic Generic Generic Generic Generic Generic Table 1
Bill of materials
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Typical performance graphs for 12V system
Figure 4
Performance graphs for 12V system
By changing the value of R2 from 1k2 to 2k2 the operating input voltage range can be adjusted from 30V to 20V, therefore the solution is able to operate from the typical operating voltage supplies of 12V and 24V for low voltage lighting. Typical performance graphs for 24V system
Figure 5
Performance graphs for 24V system
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Useful formulae for calculations
The input power from the battery during TON (assuming discontinuous operation mode) is VIN * IPEAK/2. The average input current from the battery is therefore this current multiplied by the ratio of TON to the total cycle time: I PEAK T ON ---------------- x -------------------------------2 T ON x T OFF It can be seen from this how the average battery current will increase at lower VIN as TON becomes larger compared to the fixed 1.7s TOFF. This is logical, as the fixed (approximately) LED power will require more battery current at lower battery voltage to draw the same power. The energy which is stored in the inductor equals the energy which is transferred from the inductor to the LED (assuming discontinuous operation) is: 1/2 * L1 * IPEAK2 [Joules] I PEAK x L1 T ON = -------------------------------------------( V BATT - V LED ) Therefore, when the input and the output voltage difference are greater, the LED will have more energy which will be transferred from the inductor to the LED rather than be directly obtained from the battery. If the inductor size L1 and peak current IPEAK can be calculated such that the current just reaches zero in 1.7s, then the power in the LED will not be too dependent on battery volts, since the average current in the LED will always be approximately IPEAK/2. As the battery voltage increases, the TON necessary to reach IPEAK will decrease, but the LED power will be substantially constant and it will just draw a battery current ramping from zero to IPEAK during TON. At higher battery voltages, TON will have a lower proportional of the total cycle time, so that the average battery current at higher battery voltage will be less, such that power (and efficiency) is conserved. The forward voltage which is across the Schottky diode detracts from the efficiency. For example, assuming VF of the LED is 6V and VF of the Schottky is 0.3V, the efficiency loss of energy which is transferred from the inductor is 5%, i.e. the ratio of the Schottky forward drop to the LED forward drop. The Schottky is not in circuit during the TON period and therefore does not cause a loss, so the overall percentage loss will depend on the ratio of the TON and TOFF periods. For low battery voltages where TON is a large proportion of the cycle, the Schottky loss will not be significant. The Schottky loss will also be less significant at higher LED voltages (more LED's in series) as Schottky drop becomes a lower percentage of the total voltage.
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Corporate Headquarters Zetex Semiconductors plc Zetex Technology Park, Chadderton Oldham, OL9 9LL United Kingdom Telephone: (44) 161 622 4444 Fax: (44) 161 622 4446 hq@zetex.com
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.
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