Joule Thief Single Cell LED Driver

© Brooke Clarke 2008

I started looking into a Blocking Oscillator as one way to power a High Brightness LED from a single cell battery (Alkaline, Ni-MH or a No. 6 Dry Cell).  The early work is on the LED web page.  But it's getting more involved so am starting a new page.

Blocking Oscillators are also used in many other applications.  For example if you remove the LED and replace it with a series connected high voltage diode and capacitor and add a third winding with many turns you can generate high DC voltages like used for flash cameras.  With the 7 turn coil unit if the LED is removed the T1B pulse is about 85 volts.

Working with the Fair-Rite toroids in 43 and 73 material everything seemed to work.  But the 85 material (square B-H) did not.  To investigate why a working unit using 73 material was measured again.  I suspect that a different winding is needed with the 85 material.  Here it is.

Fig 1 Schematic	Working 73 material parts: Core: Fair-Rite 2673002402 Transistor: SA1805 LED: LTW-77HC4 R1: 1k Supply Voltage 0.64 Volts. Fig 2 Base Waveform closed loop
Fig 3 Waveform at junction of T1A-1 & R1	Fig 4 Collector waveform self oscillating
Fig 5 Emitter Current	Emitter current measurement was made by inserting a 10 Ohm resistor between the emitter and ground.  This radically changed the operating parameters of the circuit. But you can see that the current is rising during the LED off time. In this case the voltage across 10 ohms gets to 320 mv or the current gets to 32 ma. Notice that in this case the current has a linear ramp back to zero lasting about 1 us, i.e. that's the LED on time.  Then there's a delay of about 3+ us before the linear ramp up in collector current.  Flagiusz sent me an improvement: "You can speed up transistor switching time by adding capacitor (ca. 1nF) parallel to R1 resistor."

The LED is connected across the transistor Collector - Emitter junction so the collector voltage shown in Fig 4 is the same as the LED voltage.  Since the power supply is a single cell with a voltage between near zero and 1.5 Volts some boost is needed to light the white LED (Vf  nominal 3.4 Volts).  The boost comes from the collapsing magnetic field in T1.   Then the transistor is turned on and the collector to base voltage is very low  (32 mV in Fig 4).  The current in T1B starts ramping up until either the transistor saturates or the core saturates or maybe both are happening.  When the rate of increase of T1B current decreases that generates a voltage in T1A that turns off the transistor.

To estimate the collector current using the value of collector voltage.
The LED all by itself is measured first:

V	I	P
2.28	1	2
2.94	10	29
3.00	15	45
3.06	20	61
3.11	25	78
3.15	30	95

Core Current

For making calculations relating to the toroid the current through the coil is needed.
May 8 2008 (05/08/08) Triad CST206-3A 300T current transformer on order.  Expect by end of May.

Joule Thief circuit w/ Current Transformer Fig 6 Current Transformeer to measure Emitter (Collector) Current

Fig 7 Emitter Current Waveform

Current Transformer

Using the Fair-Rite 73 material transformer used above with two windings of 59 turns each (see above) with the center tap where the power supply was connected not used.
To calibrate a single wire was passed through the center and driven by the HP 33120 with a 10 V pk-pk square wave.  Since it has a 50 Ohm internal impedance the calibration current is 200 ma.  The scope displays 171 mv so the cal factor is 1.17 Volts/Amp.

This plot is not showing a linear ramp down from the peak current but instead a step.  That may be because this is not an optimum current transformer.

The current transformer is NOT DC coupled.  The DC level will average to zero.
The vertical channel has zero volts offset and is 100 mv per division. 
During the LED off time the current through T1B and the transistor C-E junction ramps up until the transistor is turned off.  That transition is the trigger point at the left edge of the scope.
Immediately after the transition the transistor current is zero which allows setting the DC levels on the waveform.  Adding 75 mv gives the DC levels.
0 ma at the left edge and ramping up to 180 ma.

Hcore = (0.4 * PI * Turns * Current)/ (effective length of core)
Hcore  = (0.4 * 3.14 * 7 * .18) / 2.18 =0.726 Oe 

The power supply is showing a current of 74 ma.
The emitter current does not include the LED current so a seperate measurement was made of the total current which turns out to have the same peak to peak value but looks more like a sawtooth (does not have as pronounced a flat part).  A linear ramp from zero current to 180 ma has an average value of 90 ma.  But this is over 32 us and the other 5 us of the period there is no current so the average current would be 32/(32+5) * 90 = 77 ma which is pretty close to the meter reading of 74 ma.

R1 sets the base drive.  Decreasing R1 increases the transistor base and collector currents and more coil current means a brighter LED.  As R1 is inecreased a point is reached where the LED is no longer on.  At this point the circuit is still oscillating but the peak voltage at the LED (same as the collector voltage) is just below the LED turn on voltage.

Q1

There are a number of special requirements for this transistor.  First off it must operate with low voltages.  Most transistors are specified to work with a collector voltage around 10 V but in order for the oscillator to start this transistor needs to have decent beta and low Vces with maybe 30 milli volts on the collector.

Depending on the circuit values the size of the reverse bias spike on the base emitter junction might kill some transistors.  A work around is to add a zener diode to protect the transistor.

If high voltages are involved the collector breakdown voltage needs some headroom.

This is the first time I've used Spice, a general-purpose circuit simulation program for nonlinear dc, nonlinear transient, and linear ac analyses.  It came from UC Berkeley.
I'm using the free version from Linear Technology called SwCADIII.  It has a a couple dozen NPN transistors built in.
After trying all the transistors (and adding the ZTX690B) here are some results along with the spice model parameters.

Note the energy stored in the inductor (that's needed to light the LED) is directly proportional to 1/2 * L * I^2.  So doubling the current does much more good than doubling the inductance.  The point where the current ramp up stops is controled by the transistor parameters.
Fig 8 Gummel-Poon schematic

The model is a "T" configuration.  So RC gets added to RE for the collector emitter junction.
In the data below the ZTX1048 has RC and RE values that are 50 to 300 times lower than those for the 2N2222.  That alone may explain why it will allow more current to flow.

4 Ohms for RC + RE for the 2N2222 which limits the current to 1 Volt / 4 = 250 ma peak less than 125 ma for a ramp.

0.032 Ohms for RC + RE for the ZTX1048 plus the 0.030 in the inductor and battery total 0.062 Ohms for a current of 16 Amps peak.  So something else is the limiting factor.

By trying all the transistors some worked much better than others.
Modified Gummel-Poon BJT Parameters

		Joule Thief Ic					990 ma	880 ma	300 ma	160 ma
	name	parameter	units	default	example	area	ZTX1048	ZTX690B	2N4401	2N2222
1	IS	transport saturation current	A	1.0e-16	1.0e-15	*	2.6E-13	1.5E-12	26.03E-12
2	BF	ideal maximum forward beta	-	100	100				4.292K	200
3	NF	forward current emission coefficient	-	1.0	1			1
4	VAF	forward Early voltage	V	infinite	200			60	90.7
5	IKF	corner for forward beta high current roll-off	A	infinite	0.01	*			.2061	.3
6	ISE	B-E leakage saturation current	A	0	1.0e-13	*		4E-13	26.03E-12
7	NE	B-E leakage emission coefficient	-	1.5	2		1.38	1.37	1.244
8	BR	ideal maximum reverse beta	-	1	0.1		300	123	1.01	3
9	NR	reverse current emission coefficient	-	1	1		1	1
10	VAR	reverse Early voltage	V	infinite	200		15	14.5
11	IKR	corner for reverse beta high current roll-off	A	infinite	0.01	*	6	1	0
12	ISC	leakage saturation current	A	0		8	1.6E-12	4E-13	0
13	NC	leakage emission coefficient	-	2	1.5		1.4	1.34	2
14	RB	zero bias base resistance		0	100	*	0.1	0.1	10	10
15	IRB	current where base resistance falls halfway to its min value	A	infinte	0.1	*
16	RBM	minimum base resistance at high currents		RB	10	*
17	RE	emitter resistance		0	1	*	0.022	0.045		1
18	RC	collector resistance		0	10	*	0.010	0.027	.5	3
19	CJE	B-E zero-bias depletion capacitance	F	0	2pF	*	559.1E-12	250E-12	24.07E-12	25E-12
20	VJE	B-E built-in potential	V	0.75	0.6		0.533	0.68	0.75
21	MJE	B-E junction exponential factor	-	0.33	0.33		0.299	0.36	.3641
22	TF	ideal forward transit time	sec	0	0.1ns		600E-12	0.77E-9	466.5E-9	400E-12
23	XTF	coefficient for bias dependence of TF	-	0					0	3
24	VTF	voltage describing VBC  dependence of TF	V	infinite					0	2
25	ITF	high-current parameter  for effect on TF	A	0		*			0	1
26	PTF	excess phase at freq=1.0/(TF*2PI) Hz	deg	0
27	CJC	B-C zero-bias depletion capacitance	F	0	2pF	*	136E-12	59E-12	11.01E-12	8E-12
28	VJC	B-C built-in potential	V	0.75	0.5		0.420	0.49	.75
29	MJC	B-C junction exponential factor	-	0.33	0.5		0.267	0.36	.3763
30	XCJC	fraction of B-C depletion capacitance  connected to internal base node	-	1
31	TR	ideal reverse transit time	sec	0	10ns		3E-9	18E-9	233.7E-9	100E-9
32	CJS	zero-bias collector-substrate capacitance	F	0	2pF	*
33	VJS	substrate junction built-in potential	V	0.75
34	MJS	substrate junction exponential factor	-	0	0.5
35	XTB	forward and reverse beta  temperature exponent	-	0				1.4	1.5	1.5
36	EG	energy gap for temperature  effect on IS	eV	1.11					1.11
37	XTI	temperature exponent for effect on IS	-	3					3
38	KF	flicker-noise coefficient	-	0
39	AF	flicker-noise exponent	-	1
40	FC	coefficient for forward-bias  depletion capacitance formula	-	0.5					.5
41	TNOM	Parameter measurement temperature	C	27	50

 Agilent has a 100+ page manual on the GP model.

vicbin posted these LED models on Candle Power Forums:
.model LuxStarW1w D(Is=2.52144e-017 Rs=0.769946 N=3.33836 Cjo=100p Iave=350m Ipk=500m mfg=Luxeon type=LED)
.model LuxStarW5w D(Is=3.2946e-017 Rs=0.774809 N=6.5979 Cjo=200p Iave=700m Ipk=1000m mfg=Luxeon type=LED)

When a Fair-Rite material type 85 (Square B-H Loop) is used with similar turns to the 73 material it does not work. 
The 85 material has an initial permeability of 900 compared to the 2500 of material 73.
Also it takes about twice the magnetic field to saturate.
I'm trying to figure out what windings need to be used to get it to work.
There are diagnostic tests that can be done.
These are more useful if you have a working unit to compare to hence the above waveforms.
9 May 2008 - I've been told that 85 material will NOT work for this application.

12 May 2008 - Using the same Fair-Rite material 73 core (p/n 2673002402) and winding the primary with 22 AWG (13") you can get 16 turns on in one layer.
This gives a 280 uH coil with about 18 milliohms resistance. (the HP 4328A can not measure this because Xl/R = about 100 and spec is <3)
The secondary (base drive) winding is 4 turns of 28 AWG (7") since that's the spool I grabbed first.  32 uH and a resistance (40 milli Ohms) far lower than the fixed resistors used in series.

The idea is to use all the first layer with heavy wire to maximize the energy stored in the primary.

Spice Model

Using the measured values for the transformer and base resistor and the ZTX690B model from Zetex and the 1 Watt LumiLED model from Candle Power Forums.
LED on time 82 us
LED peak current 698 ma
Collector peak current 711 ma  peak voltage 3.82 V

By opening the connection between T1A-1 and R1 the circuit stops oscillating.  A signal generator can be used to drive R1 (which is high enough impedance that the generator impedance does not matter) and the output from T1A-1 when terminated with 1 k Ohm to ground can be measured.