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High Voltage Switching Regulator 24V

LM5008 High Voltage Switching Regulator

What do you do when your maximum input voltage = 90V and /or linear regulator has unreasonable power dissipation? First, here are a few specifications to digest for a 24V @ 70°C ambient application:

Device Input voltage rating *Power rating Heatsink Imax @ Vmax
ua7824 40V 13W 3°C/W 810mA
LM317 24 + 35V = 59V 10W 3°C/W 285mA
LM317HV 24 + 60V = 84V 8W 3°C/W 133mA
LM5008 95V Switch mode PCB pads 350mA

Observations:

  1. The heatsink gets stinking hot perhaps 100°C and the IC junction temp is 25 to 50° hotter.
  2. The first three linear regulators are not rated for 90V.
  3. In the first three linear regulators, as the maximum input voltage rating increases, the available output current decreases.
  4. The LM317 & LM317HV devices offer the advantage over the dated ua7824 in that the maximum input voltage is equal to the output voltage plus the device voltage rating (35 and 60V respectively).
  5. Only the LM5008 switching regulator meets the maximum input voltage specification and also offers substantial output current without using a physical heatsink.

Device selection:

This was an exercise that I did 10 years ago. At the time, there were few high voltage choices available –today, there may be more, but the LM5008 has become quite popular in the meantime. It is an inexpensive buck switching regulator IC with many features like current limit and over-temperature shut down. Switching regulators have much higher efficiency and lower power dissipation than the usual linear series regulators.

Schematic of the high voltage switching regulator circuit

High Voltage Switching Regulator 24V

PCB layout screenshot

PCB ScreenShot

expresspcb.com PCB artwork file download

BOM download

BOM screenshot

BOM Screenshot

SMD technology

The LM5008 is available in only SMD (Surface Mount Device) or SMT (Surface Mount Technology). It is further complicated in that the VSSOP package has a 0.025” lead pitch. As a result, this is in no way suited for breadboard testing –the best thing to do is to ‘bite the bullet’ and design a circuit board. You may also prefer to use 0805 size resistors and capacitors to make hand assembly easier –0603 is difficult, and the 0402 size is simply an electronic dust particle.

To provide heat sinking to the tiny IC, I put copper blocks on all 4 corner leads –you will see how I did it when you poke around the expresspcb file. At the time, I had to create the IC footprint symbol –now expresspcb supports this device –both are present on the layout.

Scaling the voltage
I scaled the feedback for 24V output –you may desire a different voltage –output voltage = 2.5V * the ratio of R1:R1 + R2. Output capacitor voltage may be selected accordingly.

Electrolytic capacitors

Low ESR (Effective Series Resistance) capacitors required for both input and output –the input capacitor takes the worst beating to the low duty cycle rectangular discharge waveform –I do not recommend GP (General Purpose) capacitors.

Frequency

The schematic indicates 500kHZ. I do not know where this figure came from –all I know is that the frequency is relatively high. The switching regulator uses the constant off-time topology –off-time is 300nS –this makes the max frequency approach 3mHZ –frequency is variable, depending input voltage.

Free wheeling diode

The free-wheeling or back diode (D1) is an ultra-high speed 1A, 150V rectifier –the voltage rating of schottky rectifiers is too low for this application.

Input protection

To prevent catastrophic failure from accidents or IC failure, I used a Raychem PTC resettable fuse –the one selected has a 240V interrupting rating.

For the future

LM78S40 switching regulator IC circuit –much better for the experimenter –40V, DIP outline

Undocumented words and idioms (for our ESL friends)

bite the bullet –idiomatic phrase –non-anesthesia battlefield surgery pain management –a lead bullet provided something relatively soft to bite, rather than the victim’s own tongue…
In engineering it simply means to do /learn something new, painful and /or relatively difficult.

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