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| Is there any variable DC-DC converter suitable for pre-regulation? |
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| bloguetronica:
Hi, I've been very busy lately. Only today I've came up with a schematic. The DC-DC pre-regulator will be designed according to the "PNP tracker" configuration shown. There are some things that I want to explain about the schematic: - The first half of the OPA2703 is required for buffering, while the second half is responsible for the feedback control. The added resistors will provide a gain of four over the voltage from the AD5551 DAC, while also providing voltage sensing and lead compensation. The OPA2703 was chosen because of its low offset. This block will require 0.05% resistors to maintain the overall precision. - The current control is done via the LTC2640 DAC. The INA180 is an instrumentation amplifier specialized for current sensing, and the OPA705 works as a comparator of sorts. I didn't used an actual comparator here because I want the OPA705 to operate in its linear region. The 2N3904 will starve the base of the pass transistor. - The pass transistor is a MJD122 Darlington. Unfortunately, it is not properly drawn. - The 0.1 \$\Omega\$ resistor will make sure that the inverting input of the INA180A will not be pulled down to 0, maintaining the operation in the linear region. - I'll have to replace the OPA705 and OPA2703 with higher voltage parts, or decrease the supply voltage and also the voltage gain (overlooked that part). Edit: I will use the OPA2180 as a replacement for the OPA2703. The OPA705 is fine to use, since I can supply 5V to it. Kind regards, Samuel Lourenço |
| bloguetronica:
Hi, Been simulating the voltage regulator circuit with current limiting, and it worked as expected. VG1 is the reference that sets the output voltage, which is 4x bigger than the value of this reference voltage. On the other hand, VG2 sets the current limit to a value that corresponds to this reverence voltage divided by four. In other words: Vout = 4 x VG1 Iout = VG2 / 4 I've used the OPA703 in the simulation, instead of the OPA705 I'm going to use in the end product. The transistor at its output can be either a BJT or a MOSFET (the later should have a low Vgs). In the near future, I'll implement the DC-DC pre-regulator circuit in the simulation. So far, it worked like a charm, even when defining small output voltages and currents. Attached is a DC-DC sweep of the reference voltage VG1. You can see how the output voltage increases until the current limit is reached, because a 16 \$\Omega\$ load resistor was used, and the current limit is set to 500mA (VG2 = 2V). Kind regards, Samuel Lourenço |
| bloguetronica:
Hi, Meanwhile I did a few tweaks to the circuit. I've added a CC LED indicator that light up when the threshold current is exceeded. Basically, the output of the OPA703 goes from approx. 0V to around 600mV, which is the voltage required to bias the CC control transistor. The comparator compares that to a reference voltage of around 300mV, given by the BAT54, and lights up the LED when the CC condition is detected. I've noticed that the base current of the transistor is very stable, and stays around 60 to 70uA when the circuit is in CC mode. So, there is no need of a base transistor to limit the current, since, as expected, the negative feedback loop takes care of that. Thus, the transistor is always correctly polarized and works in the active region when passing current. Kind regards, Samuel Lourenço |
| T3sl4co1l:
An op-amp driving a base directly? Oh dear. Tim |
| bloguetronica:
--- Quote from: T3sl4co1l on September 25, 2018, 02:09:29 pm ---An op-amp driving a base directly? Oh dear. Tim --- End quote --- Hi Tim, What will happen? I'm trusting the simulation too much? A base resistor doesn't seem to make a difference there, since the current is too small. Do you think it might over-saturate the transistor in case of a transient, case I don't use a resistor? Or is is just a convention? Kind regards, Samuel Lourenço |
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