When using a laptop in an area without electricity, we would normally have to rely on the integrated battery, which at best would last a couple of hours.
So, if we want to use it for an extended period of time, we will need to connect it to a power source.
Feature of This Circuit
- Input Voltage : 12V 10A
- Minimum Output Voltage : 13.6V
- Maximum Output Voltage : 26.3V
- Selector Voltage : 18V, 20V, 22V, 24V
- Maximum Output Current : 4A-5A
In this case, we would use a solar cell and battery system. However, that system alone can only supply a voltage of 12V, and according to the laptop’s charger, it requires a voltage of 19V 3.8A to charge. Therefore, we need to find a way to increase the battery voltage.
The best way to increase battery voltage is to use a boost converter, which we would often choose the MC34063 because it is easy-to-use and inexpensive. However, the MC34063 by itself can only supply a maximum current of 0.5A.
Thus, to amplify the current, normally we would use a power transistor, but this time we will use the MOSFET instead, because it works well with high frequency, can drive high current, and is relatively easy to use.
The Circuit Structure
I got this boost converter circuit from a local electronic store. It can convert a 12V input to 18V, 20V, 22V, or 24V outputs, which is in line with the initial requirements we set earlier.
But I still think it would be interesting to learn how the circuit works, so let’s start with the block diagram.
The Protection & Softstart
A circuit that has high current should also have an adequate level of protection, especially against shortcircuits, which can cause significant damage to the IC and other semiconductors.
(1) When supplied with the correct polarity of current, the positive current enters the circuit through the D2 as a base bias current for the Q3, causing the Q3 to turn on and function similarly to a closed switch. (2) As a result, current flows through the relay’s coil, causing contacts C and NO to close. (3) Allowing the remainder of the current to flow to the boost coverter circuit. Meanwhile, the LED1 lit up, indicating that the circuit was working.
The circuit would not let current through if the following conditions were met:
- Vin is less than 9V; the voltage is too low, causing the relay coil to not be able to pull in the contacts.
- Vin is reversed polarity; the negative voltage cannot pass through D2 or it received reverse bias.
Basic MC34063 Switching Regulator
Now we moved on to the basic MC34063 switching regulator part. It consists of a few important components, such as C1, C4, D1, L1, and the MC34063 itself.
C3 helps keep the current smooth during a spike in load current; without it, the load might get damaged. C2 and R3 filter the current for the IC1, thus improving the overall efficiency of the circuit.
The External Current Booster
Currently, the circuit can only supply a maximum current of 500mA. So, we will have to use an external current booster; it will amplify the current to around 4A, which is what we need.
In the above circuit, Q1 and Q2 work together as a push-pull preamplifier, which increases the effciency of amplifying the AC signal. The preamplifier is then connected directly to G of the MOSFET, thus allowing better amplifaction of high-frequency signals. Also, the MOSFET biases with voltage, meaning that it can drive current through L1 and D1 at its maximum potential.
The L1 is 0.03mH or 30uH, capable of withstanding a current of 5A. The D1 is BA550A, which can withstand 5A current and 150V.
While the Q4 is IRF3205, it has the highest drain-source (VDS) of 55V and continuous drain current of 110A. This MOSFET is often found in an inverter, and it is also inexpensive. Lastly, R2 and R4 are there to increase the stability of the circuit.
Voltage Selector
Next is the resistor voltage selector circuit; it is used to set the output voltage of the circuit. It can select between 18V, 20V, 22V, and 24V by using J1 through J4. You can also use the VR1 to fine-tune it further.
For instance, if we jump the J1, the output voltage will be about 18V, and we can use the VR1 to adjust it to any voltage within the 18V range. The lowest output voltage is 13.8V, while the highest is 26V, which means that we can set it to match the laptop power supply of 19V.
Complete Circuit
Now we combine every part together into a complete High Current 12V to 15V-24V Boost Converter Circuit.
Testing the Circuit
We test this circuit by using it to power an old laptop with a 12V/7A battery as a power source. We selected the J1 (18V) jumper and adjusted the VR1 until getting the 19V that the laptop needs. Then plug it in to the laptop; it works quite well, though the output voltage did decrease by a little bit to 18.91V.
We want to know the signal wave between the D and S of the MOSFET, as it can indicate if the switching circuit is working correctly. When the load requires a lot of current, the frequency will increase as well as the signal level (Vp-p); the result we got is a frequency of 30kHz to 100kHz, as shown below.
When the circuit is operational, there are two notable heat-generating components, which are Q4 and L1. In addition, measuring the L1 inductor yields a value of 33uH and a resistance of 0.2ohm.
Conclusion
In conclusion, this circuit works quite well; it is suitable for increasing a voltage of around 11-13V to 24V, or at not more than 26V, while powering a load of less than 5A. It is also a great example of an easy MOSFET circuit that can handle a high-frequency signal. In the future, we may be able to, for example, further increase the voltage by utilizing multiple MOSFETs connected together in parallel. We hope that this circuit will be of use to you. Thank you for reading.
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I love electronics. I have been learning about them through creating simple electronic circuits or small projects. And now I am also having my children do the same. Nevertheless, I hope you found the experiences we shared on this site useful and fulfilling.