Roman Pavelka ChaoticRoman@seznam.cz
Revision history:
- 2015-04-12, RP: Initial version
- 2015-04-13, RP: Goals refinement, requirements added.
- 2015-04-15, RP: requirements modified, commercial designs, designing
- 2015-04-17, RP: NPN and opAmp designs, test platform
- 2015-04-20, RP: requirement RM5 added, requirements release 1
- 2015-04-20, RP: requirement review and modifications
The aim of this effort is to choose simple and reliable LED driver design. Both linear and switched mode-power supplies (SMPS) circuits and commercial modules should be considered ranging from simple linear NPN transistor fixed-current driver to digitally controlled buck and boost SMPS topologies. LED driver is to be used in multi-channel configuration and design should emphasize reliability, efficiency, output ripple, price and size. Input voltage should include 24V DC for safety and reasonable efficiency.
- RF1: Proposed devices should be able to operate from adjustable 12-32V DC power supply of parameters of Meanwell RS-150-24 power supply but additional small linear power supplies are allowed.
- RF2: Proposed devices should deliver current-limited DC adjustable up to 1.25A to the single load.
- RF3: Device shall feature shutdown i.e. high power LED is OFF even in darkness for naked eye. Shutdown should be commanded by TTL signal.
- RM1: Commercial LED-drivers should be evaluated.
- RM2: Linear and SMPS topologies should be designed and studied, at least:
- linear transistor-based LED driver
- linear LED driver based on operational amplifiers
- switched LED driver based on buck non-isolated SMPS topology.
- RM3: The report to be delivered shall describe compared solutions, parameters
of interest are:
- input voltage range
- over-current protection (OCP)
- safe operating area
- thermal protection
- inrush current
- failure modes, reliability
- current rating
- efficiency
- line and load regulation
- over-voltage protection
- output voltage/current adjustment
- input/output voltage/current diagnostics
- price
- size
- RM4: Single design should be chosen, tested and characterized. KiCad schematics and example layout, BOM, budget and proof-of-concept self-consistent 6-channel device shall be delivered.
- RM5: Test tools for measurement of inrush current, current rating, regulation, efficiency and possible adjustment and diagnostics precision, uniformity and stability should be designed, constructed and verified.
This is basic idea of NPN current regulator
- Very simple and very cheap.
- Short circuit leads to catastrophic Vin*Iout power dissipation, complex to solve.
- Minimal sensing value ca 0.5V means 0.5W@1A loss here.
- Adjustment is quite difficult.
- Can be tuned for efficiency>90% for very similar voltages in all channels but only in range of current setting dependent of main DC power supply adjustment.
Simple design to study stability of linear designs is presented in following figure. Basic questions aim proper values of capacitors on input (stability vs. inrush current) and output (voltage vs. current stability). Effect of intended soft-start capacitor C2 should be tested and verified. Current regulation and its spread for different Q3 NPNs should be studied. Failure modes and line/load regulation shall be studied.
This is basic idea of Operational amplifier based current regulator, but such naive implementation will suffer of horrible error from ground voltage differences.
Following design use one differential amplifier for current sensing, another differential amplifier for current setting and improved output stage for simpler voltage diagnostics.
- Simple, cheap, efficient.
- Short circuit leads to catastrophic Vin*Iout power dissipation, can be solved by diagnostics and digital shutdown.
- Sensing value ca 100mV means 100mW@1A loss here, OK for 1% 1206 1/4W resistor.
- Adjustment is simple by potentiometer, trimmer or DAC.
- Can be tuned for efficiency>90% for very similar voltages in all channels and only range of intensity settings.
TL494 is basic SMPS chip usable in many different SMPS topologies. TL494 is used in many desktop-computer power supplies.
Buck circuit from datasheet.
P.A.R.D. = Periodic and Random Deviation noise
LM2576T is 50 kHz integrated solution available for well-under 1$ per piece.
Following commercial SMPS modules should be evaluated:
- Meanwell LDD-1000L
- Chinese drivers based on LM2596S available on ebay:
Test platform should be able to provide 24V input voltage, shutdown signal, adjustment PWM and analog signal and test load. Signal injection to input voltage and load switching should be possible with adjustable frequency and amplitude. UI should be independent on Personal computer. There should be place to accommodate device under test, namely its power transistor and an appropriate fan. The device shall feature over-current protection.
