#315 MT3608 Variable Boost

Testing the canonical variable boost circuit using the MT3608 High Efficiency Step Up Converter.

Build

Notes

The MT3608 (parts also produced as the B6286) is a very efficient boost converter that can deliver up to 24V at 4A. It requires only 6 external passive components, and is readily available as a complete module for as little as $0.40.

In this project, I wanted to build up the standard variable converter circuit from scratch and verify its performance.

WARNING: all these boards share a common circuit design that is only really safe if the input voltage is below about 7V, since otherwise it is possible to push the FB pin beyond the rated 6V when the pot is turned to the extreme. See LEAP#642 MT3608 Safe Control for an alternative that is safe for higher input voltages.

MT3608

2V to 24V Input Voltage
1.2MHz Fixed Switching Frequency
Internal 4A Switch Current Limit
Internal Compensation
Up to 28V Output Voltage
Automatic Pulse Frequency Modulation Mode at Light Loads
up to 97% Efficiency

MT3608_functional

Circuit Layout

The M3608 comes in a miniscule SOT23-6 package. If I was building this circuit for a real application, I would design an SMD custom PCB. I might still do that, but it would very much be purely for education & fun, as the total cost would probably be over 50 times commercially available modules.

So for this build I’m going ugly style on a hand-prepared copper PCB.

Component Selection

Inductor

The datasheet recommends a 4.7µH to 22µH inductor with low core loss at 1.2MHz. I’m using a 22µH CDRH104R SMD power inductor.

Filter Capacitors

22µF input and output ceramic capacitors are recommended. I don’t have any ceramics of that capacity on hand, so I’m using through-hole electrolytics. They are rated for 50V.

Diode Selection

A low forward-voltage schottky diode is recommended. Seems like a 1N5819 would be a good choice, but I don’t have any available right now. I’m using a 1N4148 instead - not ideal, but satisfactory as reverse breakdown voltage is sufficiently high at 75V.

Feedback Resistors

The feedback voltage-divider with two resistors establishes the output voltage level where Vref is 0.6V:

Vout = Vref * (1 + R1/R2)

I chose a 2.2kΩ for R2 fairly arbitrarily, mainly because I’ve seen that used in MT3608 modules. Adding a 100kΩ variable resistor for R1 means a theoretical output voltage range of 0.6V to 27.9V.

The lowest output voltage is in fact not that low. In practice it would be limited to around 1 diode drop less than the input voltage.

Enable Pin Connection

The enable pin is active high. If the chip should always be on, the pin can be connected directly to the input voltage. Since I want to test the enable functionality, I connect it with a 10kΩ pull-up resistor, so I can ground the enable pin to turn off the output.

When running with Vin=4.97V and Vout=16V, the enable pin is pulled-up to Vin with a 10kΩ resistor.

Grounding (pulling down) the enable pin disables the converter. But this does not cut output. It just removes the voltage boost, and output passes directly through the inductor and diode to output (less a diode drop).

With Vin=4.97V, Vout drops to 4.40V when disabled (given my choice of diode).

Note: one of the disadvantages of most commercial modules is that they tie the enable pin to Vin so the enable functionality is not available (without cutting and patching the board traces).

Performance - Build #1

I’m not getting efficiencies anywhere near what the datasheet claims, but I suspect this is mainly due to the low currents I am testing at. My component selection might be responsible for most of the losses: the forward voltage of the diode, and I’m not sure how well the electrolytic capacitors perform at 1.2MHz.

Load	Vin(V)	Iin(mA)	Pin(mW)	Vout(V)	Iout(mA)	Pout(mW)	Efficiency(%)	Note
n/c	4.97	2.0		4.33	0mA		n/a	minimum Vout
n/c	4.97	2.0		28.2	0mA		n/a	maximum Vout
10kΩ	4.97	2.4	11.9	4.30	0.43	1.85	15.5%	minimum Vout
10kΩ	4.97	22.3	110.8	28.1	2.88	80.9	73%	maximum Vout
10kΩ	4.96	4.5	22.3	12.11	1.23	14.9	66.7%

The module under test with Vin=5V, Vout=12V and a 10kΩ load. Most of the current is drawn to power the voltmeter I have attached on Vout:

VariableBoost_under_test

Performance - Build #2

I received some more appropriate components (1N5819 diode and 22µF ceramic caps) and built a new board. Interestingly, results are approximately the same, although the diode does allow for a lower voltage drop.

The maximum voltage I’m getting (38.2V with a 10kΩ load) is way over spec - SW Voltage maximum is 30V. I still ran the test and everything seemed to survive the short over-voltage.

Load	Vin(V)	Iin(mA)	Pin(mW)	Vout(V)	Iout(mA)	Pout(mW)	Efficiency(%)	Note
n/c	4.96	1.0		4.75	0		n/a	minimum Vout
n/c	4.96	1.0		44.2	0		n/a	maximum Vout
10kΩ	4.96	2.75	13.6675	4.75	0.48	2.28	16.7%	minimum Vout
10kΩ	4.96	41.6	206.752	38.2	3.94	150.508	72.8%	maximum Vout
10kΩ	4.96	4.71	23.4087	12.16	1.24	15.0784	64.4%
50Ω	4.95	139.2	689.0	5.00	93.3	466.5	67.7%

Performance - Commercial Module

I compared the performance of a commercial MT3608 module from a seller on aliexpress. The module has very similar parts selection to my DIY builds:

100kΩ pot and 2.2kΩ for the feedback voltage divider
SS34 SMD schottky diode
Enable pin tied directly to Vin
22µH SMD inductor
ceramic caps - appears ~15µF on the output and the input cap appears to be higher, but I can’t measure it reliably in-circuit.

Performance is slightly better. This may be due to the layout which follows the guidelines in the datasheet very closely.

Load	Vin(V)	Iin(mA)	Pin(mW)	Vout(V)	Iout(mA)	Pout(mW)	Efficiency(%)	Note
n/c	4.96				0		n/a	minimum Vout
n/c	4.96				0		n/a	maximum Vout
10kΩ	4.96	1.27	6.3	4.81	0.489	2.352	37.3%	minimum Vout
10kΩ	4.96	21.7	107.6	27.3	2.79	76.2	70.8	maximum Vout
10kΩ	4.96	4.25	21.08	12.01	1.251	15.025	71.3%
50Ω	4.95	135.7	671.7	5.00	92.8	464	69.1%