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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 becau of the low efficiency, reliability and lifetime issues associated with Halogen bulbs.
Discusd 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 1Schematic diagram
Operation
Plea refer to the typical schematic diagram in Figure 1.
neymarOn period, T ON
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The ZXSC300 turns on Q1 until it ns 19mV (nominal) on the I SENSE pin.
The current in Q1 to reach this threshold is therefore 19mV/R1, called I PEAK.
With Q1 on, the current is drawn from the battery and pass through C1 and LED in parallel. Assume the LED drops a forward voltage V F. 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/c, V(L1) in volts and L1 in Henries.
The voltage drop in Q1 and R1 should be negligible, since Q1 should have a low R DS(on) and R1 always drops less than 19mV, as this is the turn-off threshold for Q1.
V IN = V F + V(L1)
T ON = I PEAK x L1/ V(L1)
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So T ON can be calculated, as the voltage across L1 is obtained by subtracting the forward LED voltage drop from V IN. Therefore, if L1 is smaller, T ON will be smaller for the same peak current I PEAK and the same battery voltage V IN. Note that, while the inductor current is ramping up to I PEAK, the current is flowing through the LED and so the average current in the LED is the sum of the ramps during the T ON ramping up period and the T OFF ramping down period.
Off period, T OFF
The T OFF of ZXSC300 and ZXSC310 is fixed internally at nominally 1.7µs. Note that, if relying on this for current ramp calculations, the limits are 1.2µs min., 3.2µs max.
In order to minimize the conductive loss and switching loss, T ON should not be much smaller than T OFF. Very high switching frequencies cau high dv/dt and it is recommended that the ZXSC300 and 310 are operated only up to 200 kHz. Given the fixed T OFF of 1.7µs, this gives a T ON of (5µs -1.7µs) = 3.3µs minimum. However, this is not an absolute limitation and the devices have been operated at 2 or 3 times this frequency, but conversion efficiency can suffer under the conditions.
During T OFF, 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:
½ x L x I PEAK2 [Joules]
Continuous and discontinuous modes (and average LED current)
If T OFF is exactly the time required for the current to reach zero, the average current in the LED will be I PEAK/2. In practice, the current might reach zero before T OFF is complete and the average current will be less becau 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 1.7µs, but instead falls to a value of I MIN , then the device is said to be in ‘continuous’ mode. The LED current will ramp up and down between I MIN and I PEAK (probably at different di/dt rates) and the average LED current will therefore be the average of I PEAK and I MIN , as shown in Figure 3.
Figure 3
Design example
(Refer to Figure 1 and Table 1)Input = V IN = 12V
LED forward drop = V LED = 9.6V V IN = V LED +V L
Therefore V L = (12 - 9.6) = 2.4The peak current = V SENSE / R1 (R1 is R SENSE ) = 24mV/50mR = 480mA T ON = I PEAK x L1/V(L1)The equations make the approximation that the LED forward drop is constant throughout the current ramp. In fact it will increa with current, but they still enable design calculations to be made within the tolerances of the components ud in a practical circuit. Also, the difference between V IN and V LED is small compared to either of them, so the 6.2µs ramp time will be fairly dependent on the 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:
T ON 680mAx22µH
2.4
--------------------------------------- 6.2µs
=TDIS 680mAx22µH
9.60.3+()
--------------------------------------- 1.5µs
=
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As the T OFF period is nominally 1.7µs, the current should have time to reach zero. However, 1.5µs is rather clo to 1.7µs 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, becau of the peak current measurement and switch-off, it is not possible to get the dangerous ‘inductor staircasing’ which occurs in converters with fixed T ON times. The current can never exceed I PEAK, so even if it starts from a finite value (i.e. continuous mode) it will not exceed the I PEAK. The LED c
urrent will therefore be approximately the average of 680mA and zero = 340mA (it will not be exactly the average, becau there is a 200ns period at zero current, but this is small compared with the I PEAK and component tolerances).美国的饮食习惯
Ref Value Part number Manufacturer Contact details Commentsemployment
com LED Driver in SOT23-5 Q1ZXMN6A07F com N-channel MOSFET in
SOT23
D11A / 40V com1A Schottky diode in
SOT23
D26V8Generic Generic6V8 Zener diode
L122H
外语专业R150m⍀Generic Generic0805 size
提出问题
R21k2⍀Generic Generic0805 size
C1100F/25V Generic Generic
C21F/10V Generic Generic
C3 2.2F/25V Generic Generic
Table 1Bill of materials
AN44 Typical performance graphs for 12V system
Figure 4Performance graphs for 12V system
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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 5Performance graphs for 24V system
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Uful formulae for calculations
The input power from the battery during TON (assuming discontinuous operation mode) is V IN *I PEAK /2. The average input current from the battery is therefore this current multiplied by the ratio of T ON to the total cycle time:
It can be en from this how the average battery current will increa at lower V IN as T ON becomes larger compared to the fixed 1.7µs T OFF . 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:½ * L1 * I PEAK 2 [Joules]
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 I PEAK can be calculated such that the current just reaches zero in 1.7µs, 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 I PEAK /2.
As the battery voltage increas, the T ON necessary to reach I PEAK will decrea, but the LED power will be substantially constant and it will just draw a battery current ramping from zero to I PEAK during T ON . At higher battery voltages, T ON 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 conrved.
fleshThe forward voltage which is across the Schottky diode detracts from the efficiency. For example,assuming V F of the LED is 6V and V F 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 T ON period and therefore does not cau a loss, so the overall percentage loss will depend on the ratio of the T ON and T OFF periods. For low battery voltages where T ON 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 ries) as Schottky drop becomes a lower percentage of the total voltage.
I PEAK 2----------------T ON T ON T OFF
×---------------------------------×T ON I PEAK L1
×V BATT V LED –()
--------------------------------------------=元器件交易网
