Building a 112.5W boost LED driver for driving long strings of LEDs

This reference design is used to provide a high voltage boost current source for long strings of LEDs, which are not limited to street lights and parking lot lighting . Long strings of LEDs allow for cost-effective LED driver solutions, and because each LED has the same current, brightness changes can be well controlled. This design uses a 24V input and can provide up to 75V LED driver output to drive a 1.5A LED string (or multiple strings in parallel). The measured input power is 115.49W and the output power is 111.6W, with 96.6% efficiency.

Figure 2. LED driver schematic

Figure 3. LED driver layout

PCB

The MAX16834 boost design uses a common two-layer printed circuit board (PCB) (Figures 1 and 3). Some PCB functional requirements are optional and were not assembled during testing. They are marked as "no-pop" in the schematic (Figure 2). The circuit board has a ground island under the IC and is connected to the power ground at a single point to ensure low noise characteristics. Since many street lamp manufacturers do not have appropriate soldering equipment to solder other forms of packages, such as TQFN packages, this design uses a TSSOP package IC. Figure 4 shows the bill of materials for this design.

Figure 4. Bill of Materials

Figure 5. The spreadsheet provides the peak and RMS currents for the MOSFET and inductor.

Topology

The design uses a boost regulator operating in continuous mode at 200kHz. The table in Figure 5 shows the RMS and peak currents of the MOSFET and inductor. The continuous mode design keeps the MOSFET current and inductor current small. However, since the current flows through the output diode (D2) during the on-time of the MOSFET (Q1), the reverse recovery loss of the output diode is large and may cause larger turn-off noise. From the circuit waveforms in Figure 6, it can be seen that when the duty cycle is 69%, the turn-on time of the MOSFET is about 3.4μs and the turn-off time is about 1.5μs. Once the MOSFET is turned off, the drain voltage will rise to the sum of the output voltage and the Schottky diode voltage drop.

Figure 7. Output voltage (AC coupled) and voltage across the switching MOSFET current-sense resistor.

MOSFET Driver

Because of the continuous-mode design, the MOSFET and inductor peak currents are lower than those in discontinuous-mode operation. However, since current flows through the MOSFET during both the on and off periods, the MOSFET has large switching losses during the two transitions. The MAX16834 has strong enough drive capability to fully turn on the MOSFET in 5ns and fully turn off in 10ns (Figures 8 and 9), keeping the temperature rise low. If EMI is a problem in the design, change the series resistor R5 on the MOSFET gate to adjust the switching time. If this change causes excessive power dissipation, add another MOSFET Q2 in parallel with Q1 to reduce the temperature rise.

Figure 9. Drain voltage fall time

Output Capacitor

The input and output capacitors of the driver can be ceramic capacitors. Ceramic capacitors have a smaller size and work more reliably, but the capacitance is limited, especially when the rated voltage of 200V is required in the design. In Figure 5, the design table shows that the driver needs a 5.4μF capacitor to meet the output ripple voltage requirement; to reduce cost and space, this circuit uses four 1.2μF capacitors (4.8μF in total). The output voltage switching ripple is 2.88V (Figures 10 and 11), and the ripple current is 182mA, which is 12% of the output current, slightly larger than the 10% target parameter, but still able to meet the requirements.

Figure 11. LED voltage (AC coupled) and MOSFET current-sense voltage.

Dimming

The MAX16834 provides excellent dimming. When PWMDIM (pin 12) is low, three actions occur: First, the gate drive (NDRV, pin 15) of the switching MOSFET Q1 goes low to prevent extra energy from being delivered to the LED string; second, the gate drive (DIMOUT, pin 20) of the dimming MOSFET Q4 goes low, reducing the LED string current and keeping the output capacitor voltage fixed; finally, to keep the compensation capacitor at a steady-state voltage, COMP (pin 5) goes high impedance to ensure that the IC starts up immediately with the correct duty cycle when PWMDIM returns high. Each action allows very short PWM on-times, thus providing high dimming ratios.

Reducing the on-time is mainly limited by the charging time of the inductor. Referring to Figures 12 and 13, it can be seen that the current follows the DIM pulse very well. There is a decay at the beginning of the current pulse, mainly due to the ramp-up of the inductor current (about 12μs or 2–3 switching cycles). Looking at the waveform, it can be seen that it takes about 40μs to 50μs for the voltage to fully recover and build up. If the DIM on-pulse is less than 50μs, the output voltage will not have enough time at the beginning of the next off-pulse. This phenomenon will continue until the DIM duty cycle is increased. Therefore, at full load (1.5A), the DIM on-pulse should not be less than 50μs. This means that the dimming ratio is 200:1 at a 100Hz DIM frequency. The only way to reduce the minimum on-pulse is to increase the output capacitor, which will increase the cost of the system and is not necessary in general lighting. If the LED current is reduced, the minimum on-time can be reduced and the dimming ratio can be increased. Ceramic capacitors exhibit a piezoelectric effect and some audible noise will appear during dimming. However, noise can be minimized through proper circuit board layout.

Figure 13. Dimming pulse of approximately 50 μs

OVP

In Figure 14, when the LED string is open, the MAX16834’s overvoltage protection (OVP) circuit will first shut down the driver for 400ms before restarting. Because the output capacitor is small and the inductor energy storage may produce overshoot, a 107V peak voltage setting is used (higher than the 83V design value).

Circuit adjustment and other input and output

R15 is a linear digital potentiometer that can adjust the LED current anywhere from 0A to 1.7A. The MAX16834 has an input (SYNC) that synchronizes the controller's switching frequency. The UVEN input allows external control of the driver (on/off). A low-impedance signal source at the REFIN input can override the potentiometer setting to control the driver current. For example, a microcontroller's buffered DAC can directly control the LED current through REFIN. FLT# outputs a low level when a fault occurs (such as OVP). Once the fault is removed, the signal goes high. This signal is not latched.

Temperature rise

The measured efficiency is 96.63% (VIN = 24.01V, I_IN = 1.49A, PIN = 115.49W, VLED = 74.9V, I_LED = 1.49A, POUT = 111.60W). Due to the high frequency of the circuit, the driver components do not heat up. The hottest component is the dimming MOSFET Q4, which rises by about 41°C. This rise is due to the small PCB layout and can be improved by increasing the copper area near the drain. The inductor is larger in size and has a 23°C rise, which is higher than the expected 7°C (Figure 15). The inductor seems to absorb some of the heat from the MOSFET because they share a large copper pad.

Temperature measurement

The following temperatures were obtained during actual LED load testing:

VIN:24VDC

Ambient: 16°C ΔT

L1:39°C23°C

D1:51°C35°C

Q1:51°C35°C

Q3:57°C41°C

IC:33°C17°C

Power-on steps

Connect up to 20 LEDs in series between LED+ and LED-, and connect an ammeter in series to measure the current (Note: a parallel connection can be used if the forward voltage of the LEDs is perfectly matched and/or series balancing resistors are added).

Connect a 24V, 6A power supply between VIN and GND.

Insert a shunt at connector J2.

Turn on the 24V power supply.

Adjust R15 to set the current between 0 and 1.5A.

If dimming is required, connect a PWM signal (0V to 3.3V) between DIM IN and GND.

Adjust the PWM duty cycle according to the above content to achieve dimming.

Figure 14. LED string open circuit OVP

Figure 15. Predicting the temperature rise of an inductor. The calculator is from a design support tool provided by Coilcraft.

These long strings of LEDs are widely used in street lamps and parking lot lighting. This reference design uses the MAX16834 to build a 112.5W boost LED driver to drive long strings of LEDs.

Input voltage: 24VDC ±5% (1.49A)

VLED configuration: two strings in parallel, each string consists of 19 WLEDs, and 5Ω resistors are used for current balancing. Each string current is 750mA, providing 1.5A current at 75V.

Dimming: 50μs (minimum) on-pulse, 200:1 maximum dimming ratio, 100Hz dimming frequency.

Note: This design has been verified, but has not been tested in detail, and some details need further testing.